What fish can I keep in this hard water?

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Bob Mehen answers the question of keeping fish in hard water.

Q: The tap water is ultra-hard where I live in Mallorca. I am starting my aquarium off with bottled water giving 7.8 pH, 6°GH and 5°KH. It’s about £1 for 6.5 litres so not too bad, but I may have to move on to RO/DI water later.

My tank is 126 litres and filtered with an OASE Biomaster 350. Summer temperatures are around 32°C so cooling is more of an issue than heating an aquarium.

Which Corydoras would do best in these conditions? I’d like a group of eight, all of the same species. I plan to keep them with a shoal of either Cherry barbs or Harlequin rasboras.
I was intending to go for Cardinal tetras but think the water will be too hard. What do you think?

HENRY PARTRIDGE, VIA EMAIL

A: Bob replies: Corydoras are fantastic fish and ideal for peaceful aquariums. While your water has a relatively high pH it doesn’t actually appear that hard, so it may be worth monitoring the water chemistry and seeing what the pH drops to as it matures. The high summer temperatures are a slight worry, but one of my personal favourite cory species, C. sterbai, is tolerant of elevated temperatures, which is why they’re often recommended as tank mates for Discus, and well worth considering. 

They are a very attractive species with dark slate grey markings over a white background and bright orange pectoral fins. The ever dependable, ubiquitous Bronze cory, C. aeneus is perhaps the most adaptable species and will do well in your tank’s chemistry. I think they’re a little underrated personally, with good quality fish being handsome creatures with a lovely green sheen to them. 

With regards to your mid-water suggestions, then I agree that it will be a risk trying Cardinals in your water chemistry. Wild caught fish will certainly suffer but you might have better luck with commercially raised fish which are often bred in chemistries well removed from those their wild ancestors are used to. Harlequins, Trigonostigma heteromorpha, are a similar story; while any Harlequins in the shops will almost certainly be captive bred so more adaptable, you may find that they don’t show their best colours. However, a group of Cherry barbs, Puntius titteya, should be fine. These are an aquarium staple due to their hardy nature and the gorgeous colour of mature males.