The only thing I really like about a sump is how easy they are to DIY. It seemed like a difficult task becasue you cant use cheap rectangular containers as sumps unless they are either round or of higher cost and quality as thin rectangle plastic is weak and will deform. But I figured out a way to make a wooden frame for the cheap plastic Sterilite boxes so I'm making a 15g sump tank with all the scaled inserts boxes for about $15.
Other then that I have new concerns about sumps becasue they are designed with so many bio-balls to cause de-nitrification turning nitrates into gas and if this doesn't happen as is the case with aquariums that have sand, nitrate can actually escalate out of control, and in fact I had a surprise with my 244 tanks which is usually low in nitrates at water change day, my nitrates were the highest they have ever been usually 20 PPM, last water change day 40 PPM and there is some thick brown stuff growing on my PS4 sump protein skimmer. I researched this and it is becasue I have thick sand which somehow depletes the bio balls of bacteria needed and instead of processing ammonia and nitrites it accumulates and stores nitrates between water changes which can release at any time in a crash cycle.
So to remedy this Im getting rid of the Bio balls and and any sponge and using other media like polyester, and organics like lava rock, but no more bio type media for the sump.
Besides all this becasue a sump is inherently designed for a salt aquariums requiring a skimmer, a skimmer as a collection device on a fresh water aquarium is very very inefficient IMO, and I have witnessed it first hand . If the 404 doesn't hold a candle to the FX5, a sump for a 100G freshwater aquarium no way out performs a 404, that is how feeble the intake is capturing allot of water from the wrong place IMO. However a sump has one very appealing point almost to a fault and that is that you can hook a cheap auto fill device to the sump using a simple 5g water bottle dispenser and it automatically keeps your large aquarium topped off for almost 2 weeks. To me that is motivation enough for me to DIY one, but their is no way I would use a sump in place of a canister on a freshwater tank. For salt water its a completely different story becasue a overflow skimmer is ideal for salt water due to the salt water's high density which suspends 90% of all its debris and protein on the surface ideal for skimmer collection, but completely moot on fresh which has very low density and suspends most of its debris deep below the water line out of reach of a overflow skimmer, and if you don't have an intake that can collects suspended debris effectively, you not filtering but 10-20% (tops) of tank debris over a very extended period of time.
