I know it is hard water for Corys but these are tank bred fish and the supplier kept them in london tapwater pH 8. They seemed fine for the forst 4 months or so.
The substate is sand and I'm wondering if it's gone anaeriobic somewhere? Nothing visible but Ihaven't taken everything out yet. Should I just syphon it all out and replace with gravel? Other problem is algae. It's been quite bad, I've cleaned a lot of it away but there is still brown and green alage gwoing quite profusely. Blue green was growing where the vallis leaves float on the surface so i had to cut them back severely. I don't knwo why there is so mcuh algal growth, I thought iit was just part of the tank maturing but it's been going on quite a few months now.
Most likely to be pH related given the tank has very hard water. These fishies need soft adicic water with a pH lower than 7. As your other water params are excellent, this is highly likely.
TBH, there is fiercely debated argument in fish keeping circles that tank bred species 'may' acclimatize to other pH, but given the posts on this site, I now sit on the side of the fence that says you can't mess with mother nature over millions of years of evolution. Have you considered some RO at all, to bring the tak more closer to neutral?
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Maybe it is the Ph. The remaining pair are still around and appear to be ok. I will look into RO supplies again. I might also syphon out the sand. The tank water is about 0.2 more alkaline than tap water which I don't understand.