You ought to use filtration and not just oxygenate the water as you know our style of fishkeeping, we don't spend that much time in regular enough water changes which is detrimental to yr fish health if you do not use a filter. Any type will do, top filter, under gravel, canister, hang on so long as there's media to absorb waste (filter wool), bad smells (activated carbon) and beneficial bacteria (bio rings).

As for mollies, the females are usually larger with a more rotund belly.