Prime also happens to be just about the most economical water conditioner you can get, because it's mega-concentrated so a little goes a very long way. And yes, as LinznMilly says, it's very useful for detoxifying ammonia. As well as using it for water changes, I put in a whole-tank dose any time there's any possibility that the balance of my filter bacteria could be upset, e.g. when adding new fish, when using any medications, after cleaning the filter, etc. Re Aqadvisor, yes your tank isn't currently overstocked with regard to the bioload (though you'll only have room for another 1-2 fish max, and I'm assuming you'll ultimately want more than that) but the reasons not to keep fish in a too-small tank are that it will stress them out and also potentially stunt their growth, both of which can lead to various health problems. Also, platys may fight with each other and/or bully any smaller fish if they don't have enough space. Even if they seem to be getting on fine now, that dynamic can change as they grow.