Issues with white spot cardinals

[FishKeeper]

Hi, my cardinal tetras keep getting white spot. Even after treating it with Esha 2000 and Esha exit combined it will go away and then come back but I can’t seem to permanently cure it. I dose the recommended dosage and have tried extending the dosage further but no help. The other fish don’t seem to be effected but it’s always the cardinals

It’s a 200 litre tank with 5 cardinals, 2 angels, 16 Corys and about 20 corydoras.

Ammonia 0
Nitrite 0
Nitrate around 25ppm
KH 0 (wondering if this might be the problem)
Ph 7.1 but was 6.4 last week

Water changes done 1-2 times a week 30%

[2wheelsx2]

What is the tank temperature at? I have found that I always have to elevate the tank temperate to get the ich to speed up their life cycle to treat so that I don't have a persistent case. Some of the parasite might be dormant in some of your more resistant fish (not the Cardinals) in the tank.

[FishKeeper]

It’s at 25.5 degrees c. I had increased it in the past to 28 and didn’t seem to make a lot of difference

[2wheelsx2]

I normally raise mine up to 30 C for the week of treatment and I never see the ich again. You might want to try a slightly higher temp than 28. The standard temp I have on my tanks are around 27 - 28 C though, so YMMV.

[FishKeeper]

What fish do you keep?

[2wheelsx2]

I used to have discus and kept them at 31 C, but now I have only 3 tanks. One planted and unheated. 125 gallon with mostly big Pseudacanthicus, and a 100 gallon with Scobiancistrus and tetras (cardinals, black neons and glowlights).

[FishKeeper]

Okay I’ll try raising the temp a bit