I breed angels pairs in 20 talls not much different in tank size then yours and just use a tall sponge filter (if breeding,if not breeding a HOB is fine otherwise they will lay eggs on tubes every time) Altums are mostly wild caught, I've seen tank bred once. .cost more because they are fragile (their fins) hence they are mostly collected small, very few color variations..can be challenging...they like room....they get BIG...up to ten inches in fin height or more and don't like change....like a natural habitat. .tree roots, tall plants to hide in..just like their home..do stress easily, will jump (need to use a lid) need soft water( they just do better in it), live food (will starve themselves..large caught) if you're unlucky...my experience. . ect.. More work than scalare. Besides, scalare come in lots of colors...easier to breed and get from anywhere. Leopoldi are smaller than scalare..are wild caught , not easily available, I guess not a big call for them...I've only seen available three times in the last two years . . color variations almost none . It all comes down to what you're going to do, look at them or breed them.....get what catches your eye.