The best way to deal with nitrate is by increasing water changes. Your tank is stocked very heavily, so twice weekly water changes in a cycled tank will be fine. But your tank does not appear to be cycled based on information you've given us. You need to get test readings for ammonia, nitrite, and nitrate (actual numbers) and they should be from liquid kits. Strip tests are unreliable. Many LFS use these for their free samples. Try to rehome the fry, separate the male and female platy, and rehome the molly, too.