Lost a sick swordtail, think other is close behind

[Itsnotme1988]

I have had a swordtail acting odd the last 2 wks or so and I assumed it was just because the other male was becoming more aggresive towards him. The first was constantly hiding and laying on the bottom gasping and would occasionally flick against a rock. Would come up to eat then right back to hiding. Today I found him face down, tail up dead on the bottom. Never saw clamped fins or any external parasites. I noticed yesterday the other male has started to act similarly but I thought it was because of transplanting everything from the established 29 to a new 40b (used the same filter from the 29, live plants and a couple handfuls of gravel) All other fish except one otocinclus (same symptoms, and still eating) in the tank are acting fine except my shy syno. ocellifer but I think it's hiding a bit more because of the comparably intense lighting they are in now. Neons, mollies, gourami, 3 otto's, guppies and all fry, pleco and snails seem to be okay currently.

 

[Delapool]

Hi, checking water chemistry would be first thought? Might be a bit of tank stress from changing things plus if they were squabbling too much.