Just to clarify, people always refer to sponges as "Nitrate Factories", as if they just randomly release Nitrate into your system... not true. Sponges are excellent media to harbor beneficial bacteria, which run the nitrogen cycle, so they're constantly breaking down the nutrients in the cycle: ammonia ---> nitrite ---> nitrate. This is one means where nitrate comes from, along with the fact that sponges can trap detrius and uneaten foods, and if left there, will be broken down into nitrate. If you don't have a sponge, they might just be blown around down into the sump (or elsewhere) but if not consumed will also eventually break down into nitrate as well.
Bottom line is if you clean your sponge(s) frequently, they're great at exporting nutrients and maintaining a "cleaner" system.
Per your drawing there, I would remove the one on your return pump (no one ever gets around to cleaning those). You could try removing the one in your overflow, but it might cause noises since it's HOB, and you could try removing the one in your sump if there's not a microbubble issue. Personally I would leave the one in the overflow and clean freqeuntly, unless you run a filter sock, and remove the rest.
It's important to understand how and why things work, then decide for yourself, rather than buying into comments like, "Don't do it, it's a nitrate factory".
