I want to create a very natural tank for my 12" Serrasalmus rhombeus
and over 100 tropical south american fish.

Are you sure it isn't going to end up a tank for a piranha and over 100 tasty
snacks?

Generally, it's very difficult to keep a planted tank with large fish. Even if
they don't chew up the plants, they damage them just moving around the tank.

Also, an open top tank with a piranha is a bad idea. They've been known to
knock the tops right off tanks and jump out. A tank without a cover is asking
for trouble. The bright light will be stressful for the fish, too.

I suspect you and your fish would be better off with less light, and a few
hardy plants like Anubias. Or, alternately, a nice planted tank full of small
tetras, and a separate tank for the 12" predator.



Leigh

http://www.fortunecity.com/lavender/halloween/881/