Garuf said:Being in Derbyshire and being just on the wrong side of the boarder originally the water from the taps is rock hard with almost everywhere being limestone and sandstone I imagine it's got a very high level of caco3 like mine does/did.
Crypts naturally come from rivers with solid limestone basins, I suspect that this is the origin of their adaptation, vallis too have this particular adaptation.
Another issue is that higher levels of hardness seem to impede co2 uptake so more needs to be injected.
The other thing could be to cut the water slowly with RO. This also would play well with the stress associated the HOITH disease, fish kept in water much harder than their source are under constant stress and this would explain it much better than fluctuations in co2 which most fish adapt to as similar things happen naturally. It's anecdotal but a lot of people who have struggled with it are keeping fish, particularly Amazonian cichlids, are located in hard water areas without cutting the water or using some other method to reduce ph/tds load etc.
The only time I've seen a GBR stressed by the environment that its in was when I added a male GBR to an aquarium that had just come to the end of a fishless cycle. There was no measurable ammonia/nitrite, but the bicarbonate of soda that I had been adding constantly throughout the cycling period must have become engrained into all of the surfaces in the aquarium. The fish would respond by flicking off of objects that had come into contact with the bicarbonate of soda due to the increased KH and PH value that it gives off.
I keep a log book of things like PH, KH, GH among other things that I conduct on my pond water and I can confirm that my tap water has a KH of 4dH and a GH of 6dH which isn't too hard. The GBR's that I have had thrive very well in this kind of water and I've never seen the need to go down the RO route because I think its a route that a 'mad scientist' would take with all the water filtering and conditioning that is needed. I've seen many people say that they've lost their GBR's within a week after purchase and it could be that those GBR's are dying in precisely managed RO water. They are difficult fish to keep alive and breed, but I've managed to keep a male alive for 16 months (he was about 1 yr old when I bought him from the store) in good old de-chrlorinated Derbyshire tap water.