What is your nitrate level? High nitrates are not good for invertebrates. I have lost many snails in my tank, I believe from high nitrates, my level was 80 or more. I have them under control now with a refugium. I change my filter media every day or two and empty my skimmer cup at least twice a week. I learned the hard way that nitrates build up quickly from trapped food. My puffer lets lots of food get to the filter media.
Are you using RO
water for your changes? If not your are not your are most likely adding some of the nitrates back into the tank with each change.
Like cmor1701d said, more LR
to take the the burden from your canister. Are you having a algae issue?
Skimmers are not toys, they do a specific job in your tank.
A good skimmer is not cheap but you can get one for the amount you are willing to spend. You will see the difference in your water quality before long. New ones take a few days to work correctly, some (like mine) need to be adjusted daily. Not sure why mine is go difficult to keep in tune but is is an older Kent model. I need to invest in a newer model.. Anyway back to your issue, all your tank occupants will thank you for investing in one.