I have an endlers guppy who seems to be immune to whatever's going on, and one female guppy who is surviving at the moment but I think she is infected with whatever it is.
There's also about 10 guppy fry in there as well who seem fine.
My take on it is this
The tank stock was fine for the time leading up to the new fish being added, no deaths or illnesses in the previous months.
The day after the new fish were added one of the new fish died, then during the next week the other new fish died, then my original fish started to get ill and I panicked and did the filter change which upset the cycle.
So I'm leaning towards I introduced some sort of disease or parasite then crashed or bumped the cycle. I think the disease/parasite is still there and I'm treating with NDX.
I could set up my spare tank, but as I did the filter change nearly a month ago I think by the time I get another tank ready the cycle will have recovered in the existing tank.
I think I keep going trying to get the water back in spec in the existing tank, and hope the cycle recovers
Ok so I’m no expert,but I think the fact you panicked has been the mistake,and we all make it.
I think your pH is or was the important factor here.
The new fish were in a tank with a much lower pH,so adding them to your tank basically messed them up,in other words pH shock.
Your original fish would be used to the pH level,and therefore we’re fine,but seeing the new fish die panicked you,so you did what you thought was right by changing out the filter,causing a crash,which now affects your original stock.A domino effect if you like.
That’s the only feasible thing I can think of.
I did the same thing only a week or so ago when I changed from gravel to sand.I changed it all at once and caused a mini cycle,but luckily my pH us so low it stopped the ammonia hurting the fish.Your pH is high so even a trace of ammonia can do damage.
Do a big water change,say 90% and then a 50-60% for the next few days.
Don’t even look at parameters other wise you could convince yourself it’s fine,when it might not be 24hrs later.
Have you checked the pH at source?
If it’s high then avoid catfish,don’t go trying to lower it,keep it simple.
We all make mistakes,members want to remember this