Around 6 weeks sometimes longer depends on how much new botanicals I add.. The tank was almost pitch black for 2 weeks last time because of the botanicals that was a bit to much for my liking.. I hardly do water changes or small ones wich keep the tank stable. I found that doing large water changes on sensitive fish do more harm then good and the bio load is really low, and I don't add ferts to the plants and they stay alive and growing very slowly.How long do you find that lasts until the tint starts to reduce and it needs replenishing?