Well it really depends on how much you really want to look after your axie and any others you intend purchasing.
Never assume that because its been there for 4 months that it's ok. Also, looking at your photo on other thread it does look similar to ours that had gravel ingestion; especially those glass pebbles (basically anything smaller than the size of it's head will fit it's mouth and can possibly be swallowed._
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Four of ours had been on gravel and glass pebbles for a long time with previous owners and when we got them they weren't very well; owner just put their roundness down to enjoying their food! Within hours of being on sound they each pooed 2-3 pieces of gravel. Within a couple of days, glass pebbles started to appear in the sand. I didn't think they would swallow those, until i saw it. Poor things over the past year they have gradually shrunk in girth as the pebbles and gravel exited their system. We were quite lucky but probably helped that they weren't on gravel/pebbles anymore. Just imagine the amount of pain your axie goes through as it tries to excrete it - it looks pretty painful - and leaving it on the gravel, because you have a UG Filter is not a good excuse especially if you intend getting another one. I've seen cases on this site of prolapses which can be caused through gravel ingestion leading to blockage, prolapse and subsequent death.
If you don't believe me, put your axie in the fridge for a week or so and see if it poos out any gravel. Now imagine living on it and swallowing it day in, day out, excreting and swallowing it again.
Honestly, remove both the gravel and if need be the UG Filter and find a better filter and keep it bare tankfloor or sand. Bare floor is far easier to feed live food and for cleaning. Live food won't be able to burrow (if you're using worms) into substrate.
