Hello!

I had a tiger barb who I noticed behaving strangely, he wasn't eating and the problem seemed to be he was unable to move his mouth. He'd a white chunk on the lip, I am fairly sure it wasn't whitespot. Fairly sure it was either fighting with the others or some bacterial thing.

I upped the temp a couple of degrees, added a little salt, extra water changes etc for a week. It wasn't any better or worse. I took it as a good sign it hasn't progressed by this point but figured without improvement he'd eventually starve to death.

After another few days I decided to treat with some waterlife myxazin I had, I know it is basically formaldehyde and dye but bottle claims fish/plant/filter safe. What is there to lose? I admit that this is the very concentrated pond version but I calculated my doses correctly and diluted into a jug before adding. I also admit that it seemed very concentrated going in even as I tried to mix it over the surface/filter outlet.

Well after four days out of the five I was going to try I noticed my red tailed shark and rainbow fish were very unhappy. Slime peeling off, scales peeling off. Some white lumps and dead tissue/skin? I did a big water change. A dash of salt. Checked for nitrite (only test kit I've got, came up clear)

I THINK I might have swept them into a cloud of concentrated solution and given them chemical burns. I think this is what has stripped the slime coat and caused the burns/scale loss. It has been 4 days since and they are slowly recovering but I'm feeling paranoid about if it is some insidious illness which I'm about to lose everyone to! I've had a beastly case of whitespot in another tank for a while now and it is maybe making me fearful of illness.

I'll attach some pictures, does anyone have any ideas? Anything else to look out for? I lost the tiger barb this morning. By this point he must have not eaten for two weeks. His illness or injury was no better or worse throughout this.

Nobody else seems ill or injured? I'm keeping things clean and slightly warm and salted. Feeding lightly and occasional garlic. They seem to be recovering, the rainbow fish wasn't eating for the last few days but today has started again. The RTS seems to be recovering the quickest. He is impossible to photo, the rainbow fish because of its colour changing doesn't show it ideally, but it is scale damage/raised parts.

First off, make sure you check the bottle, was there any crust on it, a whiteish flake like crud?

If there is throw it out.

Things with formaldehyde in it needs to be kept in a cool dark place. I'm not sure what combination of warm and light is necessary but the white crust means it's turned into paraformaldehyde, which is bad for you and and quite possibly deadly for the fish.

As for the shark, I think I read somewhere that formaldehyde is bad for them, but that may have been salt water only.

I figured out that the rainbow fish and red tailed shark were actually fighting. I had a shoal of 7 what gets called lake kutubu rainbowfish which after an unfortunate moving incident became a single one.

Kept alone his behaviour was kind of awful. I tried moving things around a few times which helped for a while, both fish healed up but eventually he seemed to be getting the upper hand and controlling the tank. I returned him to a store to hopefully be re-homed. He was kind of massive/alpha fish compared to the others though!