Have you tried different substrates with them?
How much room do you leave at the top for air? couple inches?
The tank my bichirs have always been in is always filled as much as I'd fill any other tank which is usually up to the black frame at the top of the tank. As for substrate, they've always been on a medium sized gravel. they would probably be better on sand or very fine gravel, but they've been just fine with medium sized gravel.
Are they fast enough to catch feeder fish? Or would i be better off sticking to something like shrimp or frozen bloodworms?
yes, they are fast enough to catch feeders if they are not overly quick. for instance, there is an lfs near me that I used to occasionally buy feeders (only rosy reds) from. the bichirs have been able to catch rosy reds with minimal trouble. breeding your own feeders is always best of course to reduce the risk of introducing diseases. they could probably catch small to medium sized platys or mollies with little trouble. they will also greedily eat frozen shrimp, frozen bloodworms and frozen beefheart. a couple of my bichirs even eat off the surface gobbling up freeze dried krill and floating cichlid pellets. they will eventually start accepting sinking pellets but it may take some time for them to get used to them. most of mine ate pellets pretty much right off the bat though.
Do you feed with the lights on or off? i've read that they are nocturnal eatters, but im not sure which would be easier for them...
both, but I usually feed with the lights off more for the plecos than for the bichirs. all of my bichirs eat out in the open with the lights on or off. even the more nocturnal, reclusive species like P. ornatipinnis come out when there's food to be found.
I plan on getting a new tank, just for the bichir.. so he'll most likely be alone or maybe with something else i find interesting down the road.. so there wont be much competition for food....
if you're going to set up a "bichir tank" then I'd get several. you could go with several different species, or try to get several delhezi. who knows, you might even be able to breed them. several species have been bred in captivity. If you don't want to go with a "bichir only" tank, then there are tons of tankmates you can keep with them. basically anything that will not fit into their mouth will be fine. you don't want to go with too many other bottom feeders, but silver dollars, rainbows, and/or medium to large sized cichlids would all work. all the cichlids I've ever kept with my bichirs have totally ignored them.
How tall are your tanks? I've read a little bit (that i could find) and the articles said that they only "need" about 12" of water.. but i wouldnt wanna invest in a tank thats 20 inches tall or so, only to fill half of it... Are they comfortable with water thats a bit deeper than that?... (i'm its prolly not a big deal, but i dont like have stressed fish - i stress enough for all of us lol)..
I used to keep them in a standard 125 (72" x 18" x 22") but then moved them to my 150 (72" x 18" x 26") and they were totally fine in either tank. the reason I moved them to the 150 is because the lighting is a bit more subdued, partially due to the tank being taller. subdued lighting will make them feel more at ease and they'll probably be more active. floating plants would also probably help.