My fish have been acting weird. I have 8 platys in a 37 gallon. They started off in this tank, then I moved them to a 10 gallon while I finished cycling, which was done last Thursday. I changed my water and then moved them back into the 37 G yesterday. Pretty quickly I noticed a lot of aggression in the 37 G. I do have one male that was a bit of a bully previously when I had 4 but once I increased the numbers he levelled out. Most of them are acting aggressively today, even female to female. I've also seen a bit of flashing. My first thought was water quality but I've been testing every 12 hours since I moved them in and get the same results: 0/0/20-40 (I changed my water last night and again this afternoon to get the nitrate levels down to 10-20. I'm going to change some out again tonight). PH is 7.4. Any thoughts? I didn't notice this behaviour in the 10 gallon but it was very densely planted. While they were in the 10 gallon, I did some changes to the 37 gallon. I did add a piece of Fluval mopani driftwood which was soaked for 10 days and then boiling water poured over and soaked. I also changed my substrate from gravel to black sand. Should I be alarmed or is it just the stress of the move?