Newbie Fish-in Cycling Tips?


I have a 20-gal tall tank (Topfin kit), with driftwood, live plants, seashells, a cuttlebone, and Fluval stratum covered with gravel. I’m attempting a fish-in cycle (I know, I know) with 3 glofish tetras and 1 mystery snail.

Tank has been set up 3 weeks, but I murdered the first batch of fish by cleaning the glass with a Scotch Brite kitchen sponge (toxic!). Still not over that. But a friend showed up with 3 fish to cheer up my kids, so I quickly did a total water change (except what was down in the substrate) and replaced the filter, and crossed all fingers and toes. We’re 1 week into the new cycle.

Thus far I’ve had no measurable nitrates, nitrites, or ammonia on strips. Just bought a master test kit and got ammonia .25, pH 7.2, and the rest zeros.

I test daily. Every 3 days, I do a 15% water change, treated with Aqueon water conditioner and dosed with Topfin Bacteria Supplement. The tank has algae growth on everything, not sure if I should worry about that right now, it’s not horrible just a greenish film. Otherwise I’m just watching and rooting for these little guys.

Anything else I should be doing to boost my chances of success?


mattgirl might have some insight on your questions, let's hope she responds.


Get some prime, it makes the little bit of ammonia you have non toxic to the fish


Fish in Nitrogen Cycle simplified

The cycle needs an ammonia source. In your case that ammonia source is your fish. They are in there eating, breathing and pooping. All those things produce ammonia. That ammonia is food for your cycle but can be deadly for the fish. It is good that you have the API Master Freshwater Test Kit. With it you can know exactly what is happening with the tank water.

A nitrogen cycle is simply growing ammonia and nitrite eating bacteria. That bacteria grows mostly on your filter media but also grows on every surface in your tank.

It takes time for the bacteria to start growing. The first bacteria is one that eats ammonia. The waste from the ammonia eating bacteria is nitrite. The second bacteria is one that eats nitrites. Then the waste from the nitrite eating bacteria is nitrate. There usually isn't another bacteria to eat the nitrates so they have to be removed with water changes. Nitrates are the final stage of the cycling process so unless you have them in your source water you probably won't see them until the nitrites rise and start to fall.

Right now the most important thing you can do to protect your fish is to keep the ammonia and nitrites (once they start showing up) as low as possible with water changes. Water changes will not hurt nor slow down the cycling process as long as you use a water conditioner in the water you are replacing and making sure the temp is close to the same as what you took out. The bacteria you are growing doesn't live in the water so changing it as often as necessary to protect the fish isn't detrimental to the cycling process. Removing a lot of the ammonia with water changes may slow the process down by a few days but is necessary to keep your fish safe. Getting the tank cycled is important but the safety of your fish has to be your top priority.

Ammonia can build up pretty fast so it is possible you will have to do water changes every day or every other day to keep it as low as possible. Let you tests be your guide. Keeping them low enough to not even show up in the test is ideal but is often difficult to achieve so try to keep it down to no more than .50

I both use and can't over stress the importance of SeaChem Prime while doing a fish in cycle. It is first and foremost a water conditioner but it has the added benefit of neutralizing low amounts of ammonia thus protecting your fish from its damaging affects yet leaving some there to feed the growing bacteria.

A fish in cycle is totally doable as long as your have a test kit, Prime, lots of time and a willingness to do as many water changes as you need to do to make sure the fish are never in danger.

Contrary to some things I have read one never has to see an ammonia spike during the cycling process. While cycling my last tank I never saw an ammonia spike because I was doing 30% or more water changes every other day for most of the cycle and every day when the nitrites spiked. I know the ammonia was there because the tank did in fact cycle from a dry tank to a complete cycle within 6 weeks with no fish losses and no bottled bacteria added.

I hope this helps. If you have more questions I will try to answer them for you.


There's nothing wrong with a fish-in cycle if you do it right. It takes frequent testing and water changes, which you're doing. Do water changes to keep combined ammonia and nitrites down to 1 ppm or below. I'd recommend waiting until ammonia and nitrites show up before doing water changes to keep food for the bacteria.

Unfortunately, using bottled bacteria will send your water readings all over the place. That's why I never use the stuff. I'd rather be able to see exactly what's going on with my water parameters so I can completely control the levels with water changes. With bottled bacteria you can't do water changes for a period of time. I know the bottles say things like "Add fish instantly," but that's not really true.

I agree that Prime water conditioner is beneficial. It will detoxify ammonia and nitrites for 24 hours or so in addition to dechlorinating.


One more thing. Now that you have your test kit it is a good idea to run all the tests on your source water. It helps to know the perimeters of the water you are using for water changes.

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