Dunno why that would be a case this tank has been lightly stalked for months until I removed the 4-5 fish that were in it and placed the colony into it and I do 50% changes on this tank weekly. This tank has a fluval 406 / a large sponge filter / a submersible filter good for 40 gals as well. There’s no reason for ammonia build up. And it’s onky my hifin green phantoms showing this my blue phantoms aren’t and neither is the single vampire pleco that’s in there