You need about 1 liter of peat for every 100 liters of water, and you need to replace it every 4-6 weeks. I personally don't think it would be as effective in a large tank, because you would need to prepare the water before each water change. It's fine in the breeding tank, as it is such a small volume of water, and I can easily pre-treat the tap water. That said, it can't do any harm. When you say hard water, do you know the kH, gH readings. Farmed fish are pretty tolerable to harder water, and as long as the water does not fluctuate they will be fine. The most important thing is clean water and clean tank.