There is such a thing as too much *flow* for some species of fish to be comfortable but aside from that you really can’t overfilter.
If your fish store will test ammonia without test strips you can use that but if they use strips it’s a gamble trusting those results. There’s someone else in one of these threads right now whose lfs
told them ammonia was fine based on test strips and it turned out to be at very toxic levels. Most likely the strips got damp and are now useless.
At a bare minimum you must be able to test for ammonia and nitrite because these are the two toxic chemicals you can test for. Often by the time you buy those two tests you’ve paid for the whole test kit which includes nitrate and ph as well. I think it’s on sale on amazon right now for less than 25 plus a 15% off coupon so it’s really not a huge splurge ... (and less then you’ll spend on medicines and treatments if the fish gets sick)
Old does not necessarily mean cycled if it’s been run empty for a while the bacteria will have died off already.