Thank you so much for the information! I use a penguin 350 & I added the sponge filter yesterday. I’m using the cartridge’s for another month until my sponge gathers what it can & I got some sponge for the filter to replace the carbon once that’s all said & done.A sponge filter will primarily give you more biological filtration. A sponge is a great place to grow beneficial bacteria. It will also give you some mechanical filtration, but it is a bit of a pain. Every few weeks, you need to squeeze the sponge filter in a bucket of tank water - or put a plastic bag around the sponge filter and squeeze it in the tank. A lot of gunk will come up and cloudy up your tank for a bit if you just squeeze it in the tank
Sponge filters are very cheap and efficient. I've got one on my 15 gallon as we speak.
However, if you have an HOB already, you may not really need to add a sponge filter. The HOB will already have sufficient beneficial bacteria to handle your tank's needs - assuming that your tank is already up, running and cycled. You can add as many filters as you want and you will still get the same amount of bacteria. The bacteria will multiply in the tank up to a point - and then stop. That point is how much 'food' there is in the tank. The food is the ammonia that your fish waste produces. You can't overproduce bacteria. There is no creating "extra".
But speaking of your HOB, what exactly do you have for filter media in the HOB? Are you using the cartridges that come with it? Those are... ok. But every time you change the cartridges out, you are throwing away your bacteria. (Also, the cartridges are wicked expensive) A lot of people like to cut up some sponge and put it in the HOB instead of using the cartridges. When you do your water changes, you can pull out the sponge in the HOB, squeeze it out in some tank water in the bucket, slot it back in and there you go. A much easier way to do tank maintenance. Or use the cartidges, clean them out in tank water and DON'T CHANGE them.
One last idea, you can put a sponge filter over the HOB intake pipe. It creates a place for bacteria to grow, and it mechanically filters all of the water going into your HOB. Good fish stores will have intake sponges that will specifically fit your pipes.