Okay so I added 2 mollies and 3 platys all at the same time..about a month ago (roughly). When my stepdaughter picked her black mollie out at the store, one of the reasons she picked him was because she liked his back fin...it was standing up and has glimmers at the tips.

Since we'd brought him home, he'd kept his top AND bottom fins folded ALL the time..to the point where I started to wonder if he was even a male like we'd asked for because he totally ignored the female dalmation mollie. I never thought the mollies acted quite right because they ignored each other completely and were only interested in eating when everyone said how horny a fish breed they are.

Couple days ago I realized that I'd been using my gravel vac wrong from the beginning of cycling even. Meaning, even tho I've done regular water changes, the tank gravel was filthy which had to have been in turn, making the water icky too.

Yesterday I did a 90% water change while vaccing...and then 3 seperate times I added more water and vac'd some more until I finally topped it up and left it be. Since I've done that the black mollie especially is acting like a completely different fish...fins back up and for the first time EVER I saw him drop his bottom fins for a couple seconds (definately male yay!) and a few minutes ago the 2 mollies were hanging out side by side..something they hadn't done.

So here's my question. Which really is normal mollie behavior...what Im seeing now, or how they were before? Honestly the black mollie actually kinda seems a bit more....aggressive. He's not picking on anyone or anything...just looks a bit more agressive with those back fins up and being livelier than he'd been. His eyes even look much better. Are mollies that picky about cleanliness also? I feel bad the tank water was that dirty and I didnt even know it...honestly only a practiced eye would have been able to tell I think because it took me SEEING how the water looks now to realize the difference. Maybe I'll actually get them to be a "pair" and get some fry if the differences are right.

What you are seeing now. Mollies are livebearers so you will be having fry often. i hope you are ready. He was probably acting that way because he was stressed by the low percent of water left after the vac. you should only remove 25% of the water so you don't stress the fish out. can you get a pic of your black fish so i can see it?

Lioness, I think you finally have some good quality water that your mollies are enjoying. Congrats on getting them what they needed. As long as they had water enough to swim in I would say no water change is really too big.