I have the perfect solution, but it is not made any longer. (I have several of these and spare parts.) Marineland makes a line of magnum canisters. The used to make one called the H.O.T. Magnum. That stood for hang On Tank. I is a 259 gph canister. It has two potential media load. on uses a basket and can hold a lot of carbon or other material. Ot it has a micron cartridge that filter out down to 10 mircones. Ad some DE and it goes lower.
It has and adapter one and plug unto the output instead of using the spraybar. The adapter has a barbed output and you can connect a vacuum hose to it. Put in the Micron cartridge and away you go. The micron catches the crap you vac. up and returns the clean water to the tank. It creates it own suction. I have two tanks which run HOTs full time and then another couple I use for spot works. I also have a box filled with parts. The problem is they changed the size of the micron carts for the rest of the Magnum line and they don't fit in the old HOTs if one uses the top and bottom gaskets for the cart. If one is willing to run without the bottom gasket, it fits and works OK.
There is a single used one on Ebay but it is $170. I think I paid about $35 new for a couple and the box of parts came with several used filters. I paid about under $100 for it years back and got 3 working filters plus what would have cost me $100+ in spare parts. Of course it was all used, but worth it.
Even though I had to press into service a 40L at almost floor level (2 inch above it) which was intended as a sump for the 125 gal above it, I do not use the Magnum vac. I use a smaller, 1 inch diameter, siphon and manage to do a decent vac. But if the tanks is super dirty, I will refill it some mid-clean in order to maintain better water pressure going into the vac. tube.
I learned years back that tanks on the floor are really hard to vac and working on them made my hips and legs very unhappy. I use a small stool to sit on to work on the rest of my under tanks. These must have at least 12 inches under the tank to the floor and close to 24 over it to the upper level tank, I only do 2 levels ever. I am too old for anything more.