I just got a new platy on Saturday, and did a water change yesterday. Afterwards, maybe a few hours later, I noticed that my new platy was staying at the top of the water by the filter water output. This happened for a long time, and I have never seen a fish do that. I checked the perameters of the water, thinking that maybe there was somehow ammonia, but there wasn't, and everything else was fine as well (nitrites: 0, nitrates: 20, pH: same as usual at 8). Eventually, the new platy came out from that spot, but I don't know if he could be sick or why else he would be doing that.
Also yesterday, I saw my other platy laying on the gravel not moving. Then, after awhile of me worrying, she began swimming around the tank energetically, looking just like she usually does if not a little bit more active.
These things are just so weird to me, because I have never seen fish do this before. Do you guys think there is a problem? Maybe not enough oxygen is reaching the water because the water goes up to the glass top? (But it always does, so that doesn't entirely make sense to me.) Is it okay to fill up the tank that much? The new platy looks healthy except for staying by the filter, but could he be sick, do you think?
Thank you for your input!
Edited to add: I also lost a platy (she died) last week, which was very surprising. Maybe this is a problem that affects only platys?