Hi - I have both products and i have used them on my Betta and my killifish.
KETAPEAT - a "balckwater" additive that makes the water soft - it does discolor the water but it cannot be used with charcoal filters which will remove it form the water (which is how you can clear it form the tank). The water does turn a nice tea color but also like tea, the leaves, or ketpeat mix, are literally steeping in the water - so they have to be removed periodically - knowing when to take it out is the trick or that water may become too "black". So far, I see that it certainly does not hurt, but I ahve also not seen it make a big difference either - it may have a greater effect on certain species of tetra, as with killifish - some require peaty water absolutley and others types, like my F. Gardneri's do very well in fresh to mildly brackish water and don't seem to notice the peat at all - it did darken their beuctifully bright colors which I missed so I stopped the ketapeat. I do not recall Ketapeat being recommended for Betta fish but ....
KETAPANG VITAL - I have used this continuously for three months and then not used it continuously for three months in order to compare - neither the Betta nor the killis appear in the slightest bit different, for the worse or the better, with or without it! Very disppointing. But, there are people who swear by it!
My take on it is that Ketapang may have more effect on fish that are not so long-bred - ones that are closer to their wild origins... :dunno
It is worth a try! Just thought I would share my experience... but I would not buy either one again as they just did not make enough of a difference to my fish.