I use AC110 mesh bags. Not sure exactly how much they hold, I would guesstimate that I put about half a litre in them. I don't put too much in them though. I don't want to pack them with too much and have some of the peat not leach. The absolute easiest source of tannins I have found is oak leaves. If you live anywhere near deciduous forests, oak leaves will provide a ton of tannins per leaf if that makes any sense. I went out a couple times and collected them from a nearby state park, boiled and added the water to the tank. One thing I forgot to mention, some brands of peat will leach fertilizers into the water if ferts are used to grow the peat. When I first started using the peat I use now, I soaked some for a day and then tested for nitrates and I recommend you do the same.