If you are going to centralize your tank system, the best way to prevent illness from spreading is to have a good ultra violet sterilizer on the system so that any water going to the tanks goes through the UV
first. I've designed systems from 100 gals to 2500 gals and they all have the same principle. Only the size of the sumps, pumps and sterilizers were different.
Having said that, for freshwater aquariums, I still believe the best way is individual tanks. FW
fish can come down with many different illnesses and need different meds for treating so by having separate tanks, you can reduce the risk of spreading and of needing to use large quantities of medications. You can also better adapt a tank's water to a particular need of a particular fish. Yes, it's more work to have separate tanks but, IMO
, it's better in the long run. Whether you use sponge filters or inside corner filters or HOBs doesn't really matter. I know stores that use each type and have very good quality fish.
As for marine fish, centralizing the system is the easiest as the fish all use pretty much the same water parameters. Again, a good sterilizer is key to keeping the systems as pure as possible. A good method is also having a qt section if you decide to import your fish directly from shippers instead of from a local wholesaler.
Hope this helps