Yes, kitty litter eventually breaks down, but, so do many other clay substrates. Yes, in a sense it does leech chemicals. Montmorillonite clays both ABsorb and ADsorb nutrients in the water column. When nutrients are high it absorbs these - when low/non-exitent it adsorbs(releases) nutrients. It also wreaks havoc on GH/KH for a while but, as mentioned in the thread I linked there are easy ways to get around this.
Stock lightly and carry a big filter. - I don't have aquariums. I have ecosystems in a glass box. - Hygrophilaholic and hoarder of Anubias.