Compatible algae eater for jack dempsey chlid?

A large Plecostomus should be OK. Please remember that both the Pleco and the Jack Dempsey are fairly large fish and so they need large accommodation. A minimum sized tank for a couple of JDs and a Pleco IMO would be around 40 to50 gals. If in doubt follow the basic rules of fish-keeping which are. :- 1 inch of fish needs at least 1 gallon of water. (more like 3 gals per inch is much better) :- Every tank must have a permanently running filter. :- Every tank must have at least 50% of its water replaced every week. Stick to the above rules and your fish stand a chance of survival. Fail to look after the fishes water as above and I can guarantee that your fish will be constantly getting sick and will live a much shorter life than it should.