Everything you need to know about fertilizers in one place


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When I decided a few weeks ago to dive into the world of live plants in my tropical freshwater aquarium, it took me a lot of research to learn how to make the conversion from my already-established, non-planted aquarium. Among the most confusing parts was fertilizers. I had a lot of questions and spent many hours researching, including making some unnecessary purchases at my LFS before I finally felt educated enough to make a solid informed decision about how best to approach the issue in my own tank.

I wanted to write up a summary of my findings in a format that could hopefully save people some time and confusion. Much of my information came from the helpful folks on this very forum. I’m far from an expert, having switched to a planted tank in early May of 2016, so if any of my information is bad, I hope the more experienced aquarists will help correct me so I can update this post.

Here’s a list of the questions I had when I first started thinking about fertilizers, and the best answers I’ve found so far.

  1. I have plants, do I need fertilizers?
  2. Which fertilizers should I buy?
  3. How much fertilizer do I use?
  4. Will fertilizers hurt my fish/shrimp/snails?
  5. Do I need roots tabs or liquid ferts?
  6. What about “Excel?”

Before I start answering these, a super quick overview of plants in general will be helpful I think.

All aquarium plants need three things to survive: light, carbon, and nutrients. Light is a whole ‘nother topic, but generally, there are three basic “categories” of planted tanks: low light, medium light, and high light. Some plants do fine in low light, but many do better in medium or high light.

Carbon is often referred to as CO2, because this is the easiest form of biologically available carbon for plants to use. CO2 (carbon dioxide) is found in the air and necessary for photosynthesis, so without it your plants won’t grow. In low light tanks, adding carbon dioxide (or other forms of carbon) may not be necessary, since a small amount of carbon dioxide from the air will naturally dissolve into your aquarium water. Generally when you increase the light level, carbon dioxide becomes more and more necessary. Tanks with, say, high lighting and a fancy CO2 injection system, for example, are often referred to as “high-tech” tanks. Inversely, aquariums with low light and no CO2 injection are usually called “low-tech.” Since a number of popular plants can do quite well in “low tech” tanks, they are sometimes called “low tech” plants - Java Moss, Java Ferns, some Anubias, Amazon Swords, etc.

Finally, there are nutrients. These are the minerals and chemicals any plant needs to grow. To further confuse things, nutrients are usually divided into two types: Macro and Micro nutrients. Macros are nutrients that plants need a LOT of. Micros are nutrients that plants only require trace amounts of. The main macronutrients for aquarium plants are Nitrogen (N), Phosphorus (P), and Potassium (K), making up the “NPK” trifecta. Micronutrients are a much longer list, including things like copper, iron, manganese, sulfur, and magnesium.

The key to having a successful planted tank is maintaining the correct balance between light, carbon, and nutrients. Usually when you go out of balance in one way or another, you’ll experience things like stunted or halted plant growth, plant discoloration and death, and the dreaded algae bloom.

I may write more at some point on the topics of light and CO2, but I’ve found them to be much easier to research on my own than nutrients and fertilizers, so for now, I’ll focus on the list of questions above.

1. Q: I have plants, do I need fertilizers?

A: Maybe. If you have fish in your aquarium, low light, and no CO2 injection, you probably don’t need fertilizers. This is especially true in lightly planted aquariums. Fish waste provides nitrogen, and decaying fish food (especially flakes) will contribute phosphorus. Potassium, as well as micronutrients, are often present in tap water. Just don’t expect your plants to grow quickly with a low-tech setup like this. I recommend trying without fertilizers first. If after a few weeks, you’re noticing signs of nutrient deficiency (a couple of great articles that deal with signs of specific nutrient deficiencies can be found and here), you may need to dose one or more of the nutrients your plants need.

However, if you have medium or high lighting, then you probably have CO2 injection of some sort as well (or at least you should!), and you will definitely want to add fertilizers to your tank.

2. Q: Which fertilizers should I buy?

A: Assuming you have concluded for one reason or another that you need to be using fertilizers. The answer to this question depends on a few factors. Are you noticing something specific, like a potassium deficiency after your first few weeks of having plants? Simple; buy a potassium supplement. Seachem makes a good one, but I’m sure there are plenty of other brands available. I don’t really have a better approach to recommend for a low tech tank other than watching for deficiencies and buying ferts accordingly.

If you’re in the realm of medium or high light, you should probably be dosing all of your macros and possibly all of your micros (unless you happen to have very hard water that contains all the micros you need). This means NPK, and whichever micros you feel you need. I can recommend Seachem Trace as a great overall source of micros, but again, I’m sure there are plenty of brands. Some micros are needed in higher quantities for certain plants (red colored plants, for example, tend to require extra iron), so many of the micros can be bought in individual bottles.

You can buy macros in bottles that contain N, P, and K. I believe Seachem makes a mix that is a lot of P and K, but less N, since your fish quite possibly supply a good portion of the amount of N needed already. Seachem also makes separate bottles of N, P, and K, in case you want to control the ratios of macros you use yourself.

Aside from various Seachem and similar products, which are liquid fertilizers that come in a bottle, you can get powdered fertilizers which are basically concentrated, dry versions of the same thing. They sound intimidating because they’re literally just chemical names (KNO3, KH2PO4, etc.). But powdered fertilizers can save you buckets of money if you’re dosing a lot, especially if you have larger aquariums. Seachem liquid ferts probably cost well over $100 a year to maintain my low/medium light, 55 gallon aquarium. However, the powdered fertilizers I bought from GreenLeafAquariums (a full spectrum of all the macros and micros I need, designed for use with the PPS Pro system) cost me about $40 after shipping, and will probably last me well over a year.

3. Q: How much fertilizer do I use?

A: This is a complicated question that depends on all sorts of stuff (such as your fish population, your tank size, how hard your water is, how much you feed the fish, how many and what types of plants you have, etc. etc.). The best information I’ve been able to find is to use the instructions on the bottle, and dial it in over time. For example, if you’re following the instructions on the bottle and dosing Nitrogen every other day, and you’re noticing your aquarium nitrates are too high (more than 20ppm) even with weekly water changes, you should dial it back on how much Nitrogen you’re dosing, since your fish are keeping up with the plants’ nitrogen requirement better than you thought.

Luckily, some folks over at another website (found here) who are much smarter than myself have simplified things quite nicely. They developed a system called PPS Pro which is as close as I could find to a “one-size-fits-all” solution for fertilizer dosing. Without going into too much details, this system involves buying a list of dry powdered fertilizers (quite inexpensive) and mixing your own bottles of Micro and Macro liquid ferts. They give you exact measurements and everything. You dose a tiny amount each day into your aquarium, theoretically exactly how much your plants need for just that day, and do a water change weekly. Done. The amounts you dose are roughly calculated using the size of your aquarium and the light levels you have. I intend to dial mine in over the first few weeks or months of using the PPS Pro system, based on the reaction of my plants and my water parameters.

I like this system because it simplifies things to adding a few drops of magic juice every morning. I can feed my fish and just feed my plants at the same time. It is easy to increase or decrease dosage as needed, and even possible to play with individual nutrient ratios if the standard recipe isn’t a perfect fit for my situation. It is also far and away the least expensive approach to fertilizing my plants I’ve found. There are other approaches to fertilizing besides PPS Pro, and if this doesn’t sound like your cup of NPK, you may want to look into “EI” fertilizing instead.

4. Q: Will fertilizers hurt my fish/shrimp/snails?

A: Like literally anything else you put into your aquarium, it’s possible to overdose on fertilizers and harm your livestock. Some fertilizers are quite harmful if overdosed (nitrogen and phosphorus can hurt fish if levels get too high), while some are pretty safe generally (such as potassium). I’m also aware that many invertebrates like shrimp and some snails are sensitive to things like copper. The best advice I can offer is to dose carefully and err on the side of not enough fertilizer. It’s much easier to slightly increase your phosphorus dose after a couple of weeks if your plants aren’t getting quite enough than it is to rescue dying fish from water you have over-fertilized. This is another benefit in my mind of the PPS Pro system. Even if I accidentally dosed twice as much nitrogen as the plants needed, everything will be back down to safe levels for my fish after 48 hours maximum. It seems like a very safe and low-risk approach to fertilizing.

5. Q: Do I need root tabs or liquid ferts?

A: You might need both. I think most plants can take in nutrients from the water at least to some extent. This makes sense for plants that you don’t actually “plant” into the substrate, like Java Ferns (tied to driftwood or rocks) or Hornwort (floating). But some plants are notorious root feeders that do much better if there are nutrients in the substrate. Amazon Swords are a good example of this, as well as many carpeting-type plants like Monte Carlo, Glosso, and Dwarf Hair Grass.

If you have heavy root feeders, and your substrate does not contain plant nutrients of itself (gravel, fluorite, and pool filter sand do not, while EcoComplete and dirt, for example, do), root tabs are a good idea. My Amazon Swords are planted into a mix of gravel and fluorite, so I have ordered root tabs and intend to add a few around the roots of my swords every month or two as needed. I’m expecting this to really help them grow faster than they would with liquid fertilizers alone. I also have Monte Carlo that I’m hoping will carpet my entire substrate after several months of growth, and I will be experimenting with root tabs there as well.

Seachem and API make good root tabs, but they can be kind of expensive. Turns out a lot of people use a standard garden-store type of fertilizer called “Osmocote Plus” and fill small gelatin pill capsules with it for a much more cost-effective “DIY” root tab. I considered making my own, but found these ones on Amazon for a really good price: At the time of writing this, mine haven’t arrived yet. I haven’t compared them side-by-side with commercial options like API and Seachem, but the reviews on Amazon look good, and I’m hopeful that these will work well.

So if you’re going to fertilize, I would start with liquid ferts and add root tabs if you have plants that are heavy root feeders like Amazon Swords.

6. Q: What about Excel?

A: If you’ve been doing any of your own research on plants, you’ve probably stumbled across people discussing Flourish Excel a number of times. I’ve seen a lot of mixed opinions on Excel, but here’s what I do know. Excel isn’t technically a fertilizer; rather it’s a carbon supplement in liquid form. If your plants are carbon deficient, but you don’t have a way to inject more CO2, Excel is another way to get the carbon levels up for your plants. Interestingly, Excel is a rather nasty chemical that you don’t want to be exposed to very much.

Since I am trying to grow a Monte Carlo carpet in my low/med-light aquarium without CO2 injection, I’m actually using Excel myself. So far it seems to be working pretty well. My Monte Carlo is green and not dying, and some of my patches have even expanded to cover a tiny bit more substrate since I planted a couple of weeks ago. I just follow the instructions on the bottle and make sure to be very careful about not spilling when I dose daily. I try to dose just before my lights come on so the carbon is ready for the plants when they start undergoing photosynthesis. When I finish dosing, regardless of whether or not I actually got any chemical on my fingers, I always immediately wash my hands thoroughly with soap and water. If you’re really worried about it, maybe get a pair of rubber gloves before handling it.

So that about concludes what I’ve learned about fertilizers in the past few weeks. Hopefully this topic can provide a single place for people to come read about ferts and get all the information they need in one place. Again, I’ll add the disclaimer that I’m far from an expert, and ask that those on the forum who are more knowledgeable than myself correct me if they see incorrect or incomplete information.

I’ll keep this post updated as I learn more and have more experience with the system I’m going to be using.
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  • Thread Starter
  • #3
Additional information on Excel:

I don't have any of this type of plant, but apparently the various species of Valisineria (vals) tend to die when you dose Excel. Some aquarists have reported successfully acclimating them to Excel by starting with a very low dose and gradually working up, however. So if you want/have Vals, be aware that Excel may or may not be a good option for your aquarium. I personally plan on switching to a DIY citric acid-based CO2 injection system which I can assemble entirely and run for several months for less than $100 before I add Vals to my tank.

Also, if you're planning on dosing Excel long-term, you may want to look into buying Metricide instead. I'm not sure what the intended function of Metricide is, but evidently it's chemically identical to Excel; just sold in much larger quantities for much less.
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It's not that Vals will die but they are sensitive to it. They can adjust to Liquid CO2 but you have to slowly acclimate them.
  • Thread Starter
  • #5
Thanks, I'll go back and edit that. Since I seem to be very close to hitting the 18,000 characters per post limit in the original post, I may have to re-catalog this topic into multiple posts over the coming months as I learn more... haha.
There are some very good low cost Co2 set ups that can be gotten on ebay.
Ive picked one up but I'm not sure about the doseages (bubble count?) for tank size/fish vs. Plant types.

What if one has plants that need X amount of co2 but the stock can't handle it?
Any luck?
Hows yer tank doing?
I really appreciate your time in writing this post! I was completely confused about the ferts (especially the excel). You have dumb-dumbed it up beautifully!
Thanks so much!!

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