Hey, guys. Sorry about being really abrupt. I've read around your community and you all seem very nice. If I'd known about this website sooner I would have introduced myself under better circumstances.
Anyway, it would really mean a heap to me if you could help me with an issue regarding my guppies. More than an issue, really. One of them is dying as I type this, and another has already passed away :(
Okay, now for the backstory. I'm an inexperienced aquarist, but I hope to really get into the hobby. I bought a 50L 2ft tank about 2 months ago. It's a tropical tank, and I've kept these fish:
2 x Longfin Danios
1 x Coral Blue Dwarf Gourami (female)
1 x Bristle-nose catfish
I had ammonia problems, at first, but I seem to have sorted those out, as the tank aged. I use tank water that I condition and put in the tank. My average water temperature is about 28 degrees Celcius, and I use a submersible filter.
Recently I purchased a Black Lancer catfish, three female guppies, one male, and a male red gourami. I also purchased some plant-life, which I put in the tank, today.
After about 4 hours of settling in, I noticed 3 of the 4 guppies were worse for wear. The longfin danios (which I knew to be aggressive) had been nipping their tails. I have isolated them from the tank (they're doing fine). One of the females is fine, the male had its tail almost completely munched off, and when I found it, the danios were circling it and nibbling at it, floating at the surface.
I called up a more experienced friend of mine, who now works at our local aquarium supplies shop. He told me to do a 50% water change, and use my pH up (sodium bicarbonate) to deal with that (about 1/4 tsp per 50L, I'm doing it gradually). After the water change, my temperature moved a few degrees south, and by then the male guppy deteriorated very quickly. It was no longer floating on the top, but the bottom. About 15 minutes later the male guppy died.
The other female guppy is currently floating on the top of the water, seems pretty active, still, and is breathing fine. However, its scales are a bit white around its midsection, and its corneas (eyes) are extremely milky, almost white. I tried nudging the fish down into the water (extremely sorry if this seems cruel, I don't know much about fish, yet), and it would swim around for a while, enjoying itself, then get tired and float back to the surface, upside-down, and skim across the surface occasionally.
I'm not sure how I can fix this issue, since the water change only seemed to have a slight effect. It's been an hour since then, and I really don't want the female guppy to die, as well. She's my favourite.
EDIT: I'm pretty sure I've figured out what happened. Since I wasn't around during the first few hours of settling, I believe the longfin danios have bullied the fish, pretty roughly. I remember a while ago they did it to a larger danios, but I thought that was because it was picking on them, too. I've long-since separated the danios, and I will keep them separate. My two other guppies have damaged tail-fins, but they'll be fine.
My favourite female guppy has an extremely deep lesion, on its gut (exposing its insides). I just want to ask if it's worth euthanizing the fish, because it will only die a painful and drawn out death, this way (even though the other fish are leaving it be).
What would be an appropriate method? I don't have access to any clove oil, or vodka. I'm considering dropping the fish into subzero temperature water, as I think this is the fastest and more humane method. Other than blunt force trauma, which is extremely traumatic for any animal.