It is very hard to overdose on Stress Zyme, you could throw the whole bottle into your 15l without any ill effects. I also personaly belve it is a waste of money, as this stuff claims to hold the benificial bacteria that keeps your filter working. It is impossible to keep this becteria alive at room temp. without oxygen and ammonia constantly available. This is not the case with stress zyme, and therefore any bacteria that was in there to start with will have died off long before it gets to you. Refrigeration is another way of keeping it alive, but few shops stock this type of product, and it is usualy more expenive than normal bacteria in the bottle products, making the product less popular with inexperience aquarists, who make the bulk of the maket for these products. Therfore the refrigerated products are not worth stocking in most LFS's.
HTH
Rabbut
