Sounds as though you might have an oxygen deficiency in the water, are you dosing co2 ?
Try Seachem Paragurad. I like it cause it doesn't require you to remove any media from your filter like other meds do.
I had the same think happen 2 weeks ago. Bought two rams and 5 rummy nose tetras from my LFS. I know that there fish usually have ICH, but I didn’t quarantine like I should have.....my fault there.
First 2 days 4 rummy nose' dies....they are fragile fish in my experience so I didn’t think anything of it. Then I noticed one of my Rams got ICH. I treated with Ich-Attack (herbal) the next day one ram and one Denison barb dies. Now the Denison was big and my favorite fish so I was pissed. I noticed that another ram looked bloated and has a fuzzy white patch on it.....this confused me.
I raised the temp to 82.....to get the ich to detach from my fish. 2 Denison barbs dead in the morning. Though oxygen def so I added 3 air stones. Ich was off the ram, another ram died. Another Denison bard died. Two additional Denison barbs had fuzzy white patches on them. Thought maybe a fungal disease…..I was wrong.
3 more fish dead. Now down from 6 to 1 Denison barb.
Water parameters all still perfect. I realized that my fish had ICH and dropsy. 50% water change. Bought Seachem Paraguard and Klanapex. Use frozen food soaked in Klanapex as well as Paraguard. Put ½ teaspoon of salt per gallon in my 125 tank….its planted but I figured that little amount won’t hurt the plants.
The paraguard almost instantly took care of the ICH. The klanapex took care of the dropsy. Pretty much when u see the white on the body for Dropsy, it’s already too late.
So ....5 Denison barbs, 3 glow light, 5 rummy nose, and 4 rams dead.........
I have now stocked up on medication and WILL NOT put fish in the tank without quarantine. Plants are good…remaining fish healthy.
I posted this so people can learn from my mistakes and maybe catch the same think before it has a massive effect on their tank.
Also I found out that the Denison barbs / rose line sharks are highly susceptible to Dropsy.....for future reference.