Lethargic fish + fin rot

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Registered Member
Original poster
Apr 2, 2019
I have a 75 gallon tank with 4 fancy goldfish and 2 plecos. The tank has been established for the last 5 or so years. Same fish and all. Over time some have passed and about 2 weeks ago I got some new tank mates (bringing us to 4 total fancies and 2 plecos)

Anyway the fish had white stuff on them about 1 week after buying new goldfish, so I assumed it was ich and likely came from the new tank mates.
I treated for ich with a commercial product. This product had warnings about if the fish became lethargic to stop use, do a 50% water change and replace all filter media.
About 3 days into treatment the fish took a downward spiral.
Changed water and the filter
Now everything is so much worse. I think I stripped my tank of beneficial bacteria and lost my cycle.
I have lost 2 goldfish and am dangerously close to losing the last 2. Plecos seem to be uneffected.
I've been testing water parameters twice daily and doing 50% water changes daily. I have been using seachem Prime and Stability.
They seem to be having some fin rot, so I've been doing daily melafix. Charcoal is removed from the filter as per melafix instructions. All other filter media is there still.

Nitrites and ammonia are consistently at 0
Nitrates have been between 10-20 and I can't get them lower for the life of me, Even with tripling prime.
pH is about 7.6 consistently

I don't know what to do. I'm at a loss and need advice. What else can I do to save my fish?

the loach

AC Members
Aug 6, 2018
So which product did you use exactly? Do you have any pictures of the fish before and after?
What is the temperature of the tank?
It isn't a good idea to mix goldfish and plecos for a number of reasons.