Platy staying on bottom is not good, but this may or not be related to the filter change, or to the ammonia level, and may be related to preexisting conditions.
In a fully cycled tank, with a self sustaining bacterial colony, there should not be ammonia.
One exception is if there is detectable ammonia in your tap water; you should test for that just to be sure. However, your dechlorinator should be neutralizing that so it becomes harmless.
I am not sure what to tell you, except this:
- this is NOT due to having refurbished the filter media. Had you plain out toss the old cartridge out and put a new one. your loss of bacterial colony would have been worse, with the same situation regarding ammonia buildup, or worse.
- I suspect that in time (a week or two) the bacterial colony will be strong enough to be functional and self sustaining. In the mean time, what I would do is continue observing and monitoring for ammonia and nitrate, and to make smallish (20-30 %) water changes as needed. In time, you shoud detect only nitrate (no nitrite, no ammonia). Eventually you will be able to change some water only once a week or so, which is a doable and an advisable frequency.
Good luck!