As for changing water, always treat new water especially from the tap, chlorine will kill your biological filter and will mess up your nitrogen cycle. Seachem Prime is highly recommended for this. I would start testing your water as well, luckily platies are very tough but ammonia is still a no-no. Seachem Prime is once again recommended if you get ammonia or nitrite problems, at least until you get your filter going. Try to get some seeded material from a friend with a good tank (sponge, ornament, plant) because fish in cycling is a pain.