Most hypans although omnivorous will rarely take veggies unless they aren't offered meat for a long time. Conditioning also requires higher protein to plump them full of eggs. 20 to 30 gal tank is fine for for breeding plecos in. A bigger tank would allow better water quality and less competition though. Often temp and ph change are key factors in triggering Spawns. If you feed them 1-2 times everyday for several weeks you should start seeing differences between a well fed pleco and a gravid pleco. Some people will try for months, some people try for years before they get or don't get a spawn from these awesome fish.