Thought tank was cycled....possibly not

I have a 20 gallon high tank with 2 guppies, 2 platies, and a molly. I've had this tank for probably about 2-3 months. When I first got it the water tested 0 on everything and then slowly some ammonia showed up, and one time I tested there were some nitrites (debatable though I'm not sure the color was actually reading nitrites) but I've never had any show up after that. Eventually, I tested positive for nitrates probably a month after that (I hadn't been shaking bottle #2 so that contributed). I thought my tank was finally cycled. I've been testing it about once or twice a week since and it shows .25 ppm Ammonia and 5 ppm Nitrates every time. The fish seem completely fine as well, with the exception of one that died of fin rot. I did a 100% water change immediately after removing him from the tank and none have shown any problems. However, I tested the water yesterday and there was a giant ammonia spike like I've never seen in my tank. 1 ppm ammonia, which was odd because my fish weren't acting lethargic or poisoned at all and I don't leave food in there to decay. I performed a 25% water change and dosed with Prime just to be sure my fish would be ok. I tested again this morning and the ammonia was at the same level as before the water change and the Nitrates were still at 5. I did some research and saw someone suggest that the tap water be tested because the nitrates may be from that, and not the cycle. Sure enough, my water's ambient Nitrate level is 5 (the tap water also is 0 for ammonia). How should I proceed from here? Do I need to get some bacteria to put in and treat this like a fish-in cycle? I'm attaching a photo of the tank for reference.

 

So a few things and I may have misunderstood the post. First, you never want to do 100% water changes. It really isn't good for your cycle and causes way more harm than good. Unless you have something major going on I wouldn't do more than about 50% weekly. There are some reasons to do bigger ones but its not common to need it.

As for Nitrates. You want nitrates to read in the tank. Some nitrates is a good thing and totally normal. You don't want ammonia and nitrites. If you have those then your tank isn't cycled.

 

So a few things and I may have misunderstood the post. First, you never want to do 100% water changes. It really isn't good for your cycle and causes way more harm than good. Unless you have something major going on I wouldn't do more than about 50% weekly. There are some reasons to do bigger ones but its not common to need it.

As for Nitrates. You want nitrates to read in the tank. Some nitrates is a good thing and totally normal. You don't want ammonia and nitrites. If you have those then your tank isn't cycled.

It wasn't exactly 100% but I was told to take most of the water out to avoid getting any of my other fish sick. I'm not worried about nitrates, I know about the cycle. My worry is that my ammonia should not be spiking like this in an established tank and due to the fact that untreated tap water tests positive for nitrates and I've never detected nitrites the entire time I've had the tank, the ones I'm detecting are not an indicator of a cycled tank. Meaning my tank has not cycled. I'm just wondering how I should go forward if I need to treat it like a brand new fish-in cycle or just keep doing what I have. 25% change every week and maintenance testing.

 

When you did your WC did u mess with your filter at all? I would stick to checking the ammonia levels and performing 50-75% WC’s to bring the levels down and dose Prime for the entire tank volume not just the % of water you change. Hope this helps… adding a few live plants could help with your water quality as well.

 

When you did your WC did u mess with your filter at all? I would stick to checking the ammonia levels and performing 50-75% WC’s to bring the levels down and dose Prime for the entire tank volume not just the % of water you change. Hope this helps… adding a few live plants could help with your water quality as well.

The filter I have is just a standard cartridge filter and I didn’t change the cartridge so no. And thank you! I’d thought about plants before but I’ve read Mollies and Platies like to munch on them and that can cause Ammonia issues that mimick over feeding so I didn’t want to run that risk. Especially since they already pick at the fake plants frequently.

 

The filter I have is just a standard cartridge filter and I didn’t change the cartridge so no. And thank you! I’d thought about plants before but I’ve read Mollies and Platies like to munch on them and that can cause Ammonia issues that mimick over feeding so I didn’t want to run that risk. Especially since they already pick at the fake plants frequently.

Good that u didn’t change the cartridge. You know not to just remove it and throw it away but instead swish it around in a bucket of old tank water? We have had Mollys and platys in the past and they have never messed wit any live plants. Maybe they eat decaying plants and they may also nibble on algae if it forms on any surfaces but as for live plants you should be safe to add some.

 

kind of a myth ( well it's a myth really) you can do 100% water change, Discus keepers/breeders will do it without issue once or twice a week to raise big fish.

There's minimal beneficial bacteria is in the water, like 100% is in your filter at the start, and after about 4 months it's established in your substrate and hard surfaces as a thin layer on or just below the surface, and then 80% is in your filter and about 20% is in your substrate if you aren't vacumming the substrate for the first few months. the first few months it can be moving about the water and settling in.

If you deep clean your filter within those first 4 months or throw out the filter pads and replace them, you are throwing out the vast majority of your beneficial bacteria (you are throwing it out if you do it any time but after 40 months the bacteria is quick to recover off what's on the tank surfaces and substrate), causing ammonia or nitrites to show up on tests because your biological filter is running at 20% or less until it builds back up, roughly doubling every 15 hours. you can dose prime or a different ammonia a nitrite binder if you get the spike and need to redose within every 48 hours so the ammonia and nitrite isn't harmful.

so with an established tank, and 20% you are at 1/5th load, in 15 hours roughly at 78F you can be at 40% then 80% at another 15 hours and then 100% again after the next reproduction. cooler water slows reproduction of bacteria down.

if your tap water is 5 nitrates, the best you will get from it on the low side is always 5 nitrates unless you have plants or another way to use them up, you can remove 100% of the water and replace it and there will still be 5 nitrates otherwise. 5 nitrates is fine, 10 nitrates is fine, 20 nitrates is time to do a water change.


At 2-3 months you should be cycled, even doing it slow and low, and worst case, a temporary spike to ammonia or nitrites can happen by changing filter cartridges. if you didn't do that, then you must have done something wrong and made a misstep. this spike can be caused by a bacteria die off in your biological filtration, an excess bio-load of fish, an abundance of algae, to much time between maintenance and water changes, or over feeding.

I suppose your cycle could have stalled somehow and it's now 2-3 months later, but that's unlikely, more like it was set back somehow.

and, is you test your tap water, let it sit for 15-30 minutes before testing it to see where it's at when it goes into the tank for an accurate test of it. I don't know what you did, or didn't do, but it could only be caused by so many things. Fumigation? cleaning chemicals from the housekeeper? some house air fresheners? These things can screw up the test or screw up the biological.... could be you disrupting the cycle trying to keep it spotless also and the bacteria not actually settling in.