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While I realise that by definition most S American catfish come from areas of lower pH I have a tank with a large piece of driftwood (unknown sp.) that sits at pH8. In it are a school of Rummy-nose Tetras who are bright and healthy, but none of the Corys I have tried have been successful and have been shipped out after the first few deaths.
A tank without catfish is no fun so I am wondering if anyone has any other suggestions?
Can't grow too big or eat plants, it's only a smaller tank.

The thing is - the continent of South America seems like it has less calcium then Europe does - but fish from outside the Amazon region can withstand higher pH. Corydoras paleatus does come from this area

The same goes for Africa and Asia - the rainforrest is more acidic then elswhere. But most exported fish come from these rainforrests

The Central American rainforrest is not than acidic, but hardly any fish is exported from there. The onese we keep are offspring from offspring - you get the meaning.

The same is said about Australia and New Guinea, the big island in the north. Perhaps you can manage to get catfish from there?

I have well water. My pH is at 8.3. I have CW021, C.Aeneus, C.Ehrhardti, C.metae, C.Paleatus, C.Panda. They all are doing great and spawning. Your 8.0 pH should not be a problem unless you want wild caught fish.

You might try hoplo or porthole cats, I have both. I also have Cory similis, aeneus, and F1 eques from a local breeder. Our tap pH is around 8. The fellow I got my eques from doesn't use RO/DI either. Tatia sp, banjo cats, bumblebee, Akysis sp. are in my collection too.

Some of the ways to lower KH in your tank are by using acid buffers, mixing different types of water, or releasing tannins (tannic acid), all methods need to be done in moderation to prevent a sudden change in pH that could lead to damage or death of aquatic life in your aquarium.

So sorry I didn't see all these kind replies! A new pH kit taught me that it wasn't actually my problem, which I now suspect was salt leaching out of a big piece of driftwood. It seems to have settled down now after time and regular water changes.
This was my first tank built here and it had to be put together with a variety of dodgy local water sources, so water was my first consideration.
Anyway, I hope that this info has been useful for some others, and thank you again.