In my old shop location I had a 1700 gallon aquarium. My filtration was a large wet / dry with approximetly 100 cu gallons of bio balls, (sump could hold about 500 gallons of water), an HSA 2000 protien skimmer, Ozotech Model II ozone generator, 2 HP Dolphin water pump and an Octopus controller. I used titanuim heaters in the sump, ground probes in the sump and in the aquarium. My lighting was 3 ice cap 660's with 9 six foot VHO's (contrary to what Ice Cap suggests, a 660 will run 3 six footers with no problems).
Are you having water quality problems? Are you sure that your lemon sharks are not what is killing the other fish you have tried? I had 1 lemon shark and it was really mean, about the only thing it didn't kill was other sharks and moray. Another concern I have for you is that a single lemmon will out grow 700 gallons not to mention two of them. I realise that they are cool espcially since you can get pups under 24" and they are a fairly hardy shark too but they get 6 to 8 feet pretty easily and 10 footers are not that uncommon. Hey, I'm not preaching here, just something to think about...
It wasn't by chance a guy named Mitch from up in Canada who built it was it?
Joel