Peat certainly is best since it’s the way nature chooses herself in the homelands of our fish. But there are three problems:
1. It is always risky to buy peat in a shop for gardening. Often, fertilizers are mixed in but not openly declared. At any rate you must avoid this.
2. Peat isn’t peat. There are many different sorts of peat. Once you buy a peat that reduces the pH very efficiently; that should be the case. But then there is peat that has no value at all; it’s filtering function is not better than using an old doorscraper. You must look for the first mentioned, but often you receive the second only declared in the highest notes of appraisal. If you buy those small rounded pellets from the pet shop trade you are mostly on the safe side; mostly.
3. The most serious point is: Peat is a product from bogs and moors. And we rightly protest against the heavy use of it in gardening. There are substitutes for it for that purpose, but the commerce goes on. But it’s a problem, too, that we protest against the destruction of the peat bogs in south-east Asia but again use peat from our countries, participating in the destruction of our peat-bogs for establishing nice aquariums.
Now, one could say: But it’s so little a quantity! Gardening with peat is a big business, but using a small amount for a Paro-tank is negligible. Well, I leave it to you to valuate this argument. I am much in favour of those aquarists who for that reason look for other methods in our hobby, too. For instance using alder-cones, or oak-leaves, or oak-extracts or even acids (not vinegar, of course!). The results of these alternative methods are good.
Nevertheless I think that really acid peat is very good indeed. Maybe there are further ingredients that we don’t really know that add to the function we are here speaking about.