I used to work for petsmart (topfin is their brand) and I would NEVER suggest this filter to anyone for this very reason. The cartridges are the same exact thing as the whisper biobags and all it is is a sponge bag with carbon inside. Bacteria will colonize on this sponge but as soon as you throw the cartridge out, you throw the bacteria away too. Whisper filters had the same issue so they made a new design with a smaller black sponge that sites in front of the biobag to colonize bacteria on. This new black sponge is meant to never be replaced. Unfortunately, I don't think they sell those anywhere and if they did, I don't know if they'd fit on the topfin filters as well. What I used to tell customers in this:
WHen you replace the filter, put the new bobag/cartridge in and then take the carbon out of the old one and place the sponge part back into the filter behind the new cartridge. That way bacteria should "transfer" from the old cartridge to the new one. There is still a problem though in that you really should rinse your cartridges once a week to remove debris but that will also harm bacteria.
The short story is...it's a poorly designed filter. Its very economical and makes for a cheap kit but I would seriously consider upgrading. Also, if you got the topfin 30 then you bought a 29 gallon kit and most people would actually go with a 50+ gallon filter on a tank that size. I personally suggest the penguin biowheel filters and i'll teach you a good petsmart trick...
Petsmart matches the prices of their own website. Generally the website prices are cheaper to compensate for shipping so larger, heavier objects are considerably cheaper. Find the product you are looking for on their website, print the page showing the model and price of the product, bring it in to the store and get a good discount
(p.s...I saved $80 on my canister filter doing this.)
I know you said you didnt want to buy a new filter but basically you have an underpowered, poorly designed one right now. The other option would be to buy a second smaller filter and put that on the tank with your topfin 30. Then, just stagger the times you change filter cartridges so you keep as much bacteria in the tank as possible at all times.
Hope that helps!