Ok, let’s take a step back here and discuss the whole filtration process.
First, mechanical filtration and biological filtration are two different things. The sole purpose of mechanical is to remove actual waste from the tank. This is why it’s best to try and change it as often as possible to remove the detritus and fish waste it catches. If the detritus caught in the mechanical filtration, but is still in the system, it’s still contributing to the bioload. Once it has been removed via changing out or cleaning the mechanical filtration (in your case, the filter sock), it cannot contribute to the bioload anymore. When I talk about bioload, I’m mainly referring to the end process nitrate amounts. Although nitrates are not really harmful to fish until they get quite high, the lower they are the better. So, removing the detritus and fish waste BEFORE it has a chance to be converted into ammonia, which becomes nitrite, which becomes nitrate, is very beneficial to keeping good water quality. Since discus have been known to be much more sensitive to nitrates than most other fish, every little bit helps in giving them optimal water quality.
Now, biological media has one purpose and that is to give the beneficial bacteria surface to grow on. The different products you see are advertising their ability to provide more surface area. Porous media has much more surface area than non porous. This is all because the beneficial bacteria grow on surfaces and NOT in the water. Therefore, the more surface area, the more beneficial bacteria can grow. This beneficial bacteria is what converts the toxic ammonia produced by fish waste and uneaten food into toxic nitrite, and then into much less toxic nitrate. Although nitrate is still toxic, fish generally do fine if the levels are kept low enough. Again though, discus are much more sensitive to these nitrate levels, so you really want to try to keep them as close to 0 as possible. There are a lot of media’s that advertise how much surface area they have and it’s easy to spend a lot on stuff like marine pure. However, the cost vs the benefit is generally not there. I usually just use the cheap ceramic rings from Amazon and have always had good luck with them. Most people, me included, don’t need nearly as much biological media as what they actually have. It is important to make sure the water passes through the media for it to work. If the media is in a low flow area, most of the water will get back to the main tank without having had the ammonia converted. For your sump, you might want to consider a small power head in your middle chamber to circulate the water through the bio media.
They way this all works together in your sump is to have the water come straight down into the mechanical filtration (your filter sock) to remove the detritus and solid waste. Then, the “clean” water passes through the biological media to allow the beneficial bacteria to convert the ammonia to nitrite, and then to nitrate. Finally, the clean and ammonia-free water is pumped back up to the main tank.