If you need it David, I have so much DE it isn't even funny - happy to give some up in the interest of science. I bought it for an in tank filter for dino control, and it came in a 40# box - I used like 12 oz.
Manual removal works, but you can put filter floss in the canister - it won't pass through that. Sounds like it is small enough to manage with normal siphoning though. When doing normal projects I usually siphon into the sump through a filter sock but for bigger things the canister I have found pretty simple as I don't need to watch for socks overflowing.
Remember to take before and after photos!
With that form factor I would personally try a horizontal mounted gyre type pump.
light spectrum was going to be my first suggestion but it looks like you are already playing with that. I agree that spectrum is probably the main culprit unless the uglies are proper predominantly in the lowest flow areas currently.