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Several months ago, I moved to new city and had to drain my 20-gallon tank and give away my fish. The tank sat empty (aside from the gravel) for several months. Last week, I decided to fill it back up, add some bacteria (Tetra brand, I think), and let it run for a couple days before adding fish. The day after I filled it, everything looked fine when I went to work. But when I got back, the water was like pea soup. I changed about 50% of the water and waited a few more days...it's still green, though not as much. Today, I changed about 25% of the water.

What I can't figure out is, why did this happen and what do I do about it? From what I've read, free floating algae is caused by a) too much light, and b) too many nutrients. But, the tank is in a dark room and the light wasn't even on that day. I've also kept the light off since this happened. I don't know where any nutrients could be coming from since there are no fish in there. My only guess is that maybe algae that had dried on the gravel is decomposing. Could that be it? And if so, do I just need to keep changing the water until it goes away? Also, is it safe to add fish, or should I wait until this is resolved?

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  • A few days might be too short for cycling the tank. After all the starting is not completed after a certain time, but if the ammonia and nitrite levels are no longer measurable. The algae might have been boosted by a nitrate peak resulting from a expansion of bacteria converting nitrite to nitrate. There're a lot of things happing during the start of the cycle and the effect might well be equivalent to throwing half a can of fish food in your tank. Smaller tanks such as yours have larger peaks of nitrogen products. Never add fish before measuring water values! Jul 22, 2018 at 17:29

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