Having no substrate is perfect for a fish store. They turnover fish like no tomorrow and gravel can hide parasites and gets filled with fish dung and with the fish load they run just become nitrate factories.
Bare bottom allows you to keep the tank clean and all the dung goes into the sump and is manually removed. When you are constantly netting fish it will stir up all sorts of particles from the gravel and look terrible for an hour every time.
To answer the question on jewel filter media. Ignore the instructions, seriously burn them.
Do not change the media unless it is falling apart which takes almost a decade. Do not bother with the nitrax or carbon pads, again they are a waste of money. If you have them already just put them in and leave them. Again don’t bother buying ceramic media unless it comes with it, it’s a waste of money and makes no difference. The size of the jewel filter media is more than sufficient for the size of tank and provides more than enough surface area for the amount of fish you can get in it. You could run the whole tank off one medium sponge filter. Run your sponges in the following order:
Course
Medium
Fine
Special(carbon, nitrax, ceramic)
Floss
The only thing you need to routinely change is the floss again don’t buy the official stuff it’s a complete rip off. Go on eBay and search for filter floss, buy a lifetime supply for less than £10 delivered. Rise (squeeze repeatedly) the sponges in old tank water when you do maintenance. You don’t need to do it every time but they will eventually clog the time is takes to do that depends on how many/size of the fish.
I am all for spending the cash where it makes a difference but special filter media is a complete rip off especially stuff like biohome.