I have successfully kept discus fish for years but I have never had any luck with breeding. We live in a hard water area and even though I filter the water through peat the water is still not really soft enough. Is it OK to use bottled mineral water which is very soft in a tank containing only discus fish?
Or what other ways of softening the water are there without using RO water so that I may breed my discus?
Many aquarists try using bottled water at times, usually when they want to make the water harder, but there can be several problems with it. As an experienced aquarist I'm sure you'll understand the importance of checking all the parameters of the water you add, but just to clarify the matter for other readers, mineral water often contains salts which can upset the balance of your aquarium.
Never use mineral water with any additional ingredients and never use carbonated mineral water (even after it's stopped being visibly fizzy). If you do use mineral water, leave it to stand overnight first with an aquarium antibiotic added, as bacteria can grow in it after bottling.
If you add soft mineral water to your aquarium you should do so very slowly, mixing it with other water, aiming to stabilise the water hardness at a lower level without rapidly changing the pH.
Your peat filtering won't have been lowering the pH because of the quantity of dissolved minerals in your water, but once you start to reduce these it will take effect. You'll need to gradually reduce the amount of peat you're using to stabilise the pH where you want it.
A simpler alternative to using bottled water is using a filter system. This will sit in your filter system and filter out dissolved minerals at a steady rate, making it much easier to maintain the right balance.
Bear in mind that you may not need to soften the water much in order to have an effect on your discus. They'll respond to the change as much as to the acidity itself. If they still seem reluctant to breed, it may be because they don't have enough room - discus need a lot of space to feel secure about breeding.
Try arranging rocks and plants to create more privacy and more secluded spaces.
Finally, you should note that discus sometimes don't breed because they just don't happen to find each other attractive. This is unfortunate, especially if you have a sizeable group, but it does happen.
Like humans, discus are picky about their mates and they tend to form lasting pairs. Softening the water may make them more frisky, but it's not guaranteed to make them take an interest in each other.