I have 6 harlequin rasboras in a 20g tank (they are in with 5 corys and 2 otos), and I have noticed over the last couple of days some of them look really fat and bloated. One of them is REALLY bloated and swims sort of..off kilter? It's still active and darts around with the others. However, I noticed that during feeding time last night and tonight it didn't eat. Just swam around like it was trying to chase food but didn't actually get any (and didn't try either). It also sort of looks like there's something fuzzy on its anal fin? But they are so fast it's hard for me to tell whether it's the light or my eyes playing tricks or there's something there.
I did a 25-30% water change on Sunday, and my water parameters right now are as follows:
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 10 ppm
noticed that a number of them keep flipping upside down and rubbing the bottom of some of my silk plant leaves with their bellies. They are also doing the same on top of the plant leaves, only they are not upside down. As I understand it, this is mating behavior (and is probably why my poor oto is sulking, as that is HIS plant)...However, the fat swollen fish is not the one flipping upside down or going on top of the plant leaves. So I am not sure what is going on. I have been watching them for about a half hour now and the bloated one has not gone by the plant. It just swims around the plant in circles, or swims alongside the others that aren't busy being upside down on the plant.
I was also slightly worried that rubbing against the leaves (for the ones that aren't upside down, but are just rubbing their bellies along the top of the plant leaves) might be cause for worry, since fish can rub stuff when they are uncomfortable/have parasites?
I tried to get a picture of the fat one, but they are fast little suckers. So I apologize for the relatively lousy pictures. I tried to get pictures of them doing whatever they're doing on the leaves but they stop when I get too close so I've had to just sit at my desk and watch them from across the room. Little buggers.
Any advice would be appreciated!