Hi ctoychik,
its good you are exploring all avenues.
some would be close minded and wouldnt admit that there is learning to be done. i for one, ask even if i am 90% sure of an answer, just so i have peace of mind. no harm in that right?? lol
canister filter:
http://www.hagen.com/uk/aquatic/manuals/fluve01.cfm
http://www.eheimasiapacific.com/prod_e_extfilter_classic.html
they are available everywhere, but they are expensive, the above brands are the top brands you can get, you can get cheap chinese knock offs. they work too, but less reliable.
Honestly, if you are investing Thousands on arowanas, catfish and other large fish - then canisters are great investment. Fluval FX5 is my best pick for value for money and effectiveness. Eheim is over priced, if you are rich, then eheim is the way to go
Sump and trickle system:
this is where your tank has an over flow. the water drops down to another tank at the bottom of the tank, in that tank is bio media, filter wool, then a return pump that pumps the water back to the top of the tank. where it over flows again. basically a sump and triple is a GIANT home made version of a canister external filter. same principle. its actualy cheaper to do this and very effective.
i think just remove the UGF completely, not as such bare bottom, a very thin layer of sandy substrate is good i think, to make the fish feel at home a little.
strange and bad bacteria would certainly be created in that black sludgy stuff, and thats probably one of the cause.
as rivers and lakes have algae blooms that kill fish, you had bacteria bloom.
they hide under the gravel and when the condition is right. there is a large accumulation of poo an stuff. then suddenly they multiply uncontrolably, and kill things.
a canister is great. all you do is open it, take out the old filter wool, and put new ones in, and you nevr have to syphon if you have bare bottom tank.
just water changes.