Sand the acrylic, (to roughen the surface to make the paint adhere), then paint with flat latex paint. Seal the paint with Polygem 1618 -- clear epoxy coating. Make sure you use flat paint or the 1618 will not adhere properly to the paint.
1618 is an extremely clear epoxy coating for sealing or beautifying most surfaces.
polygem.com
If you want, I can give you the name and phone number for the Polygem technical support guy.
This would totally work for the floor and back of the tank. He advised me that I could do it on the floor of my tank, unfortunately there is an ugly join on the floor of my tank and the joint of the front panel and the floor has globs of stuff...tank manufacturer figured it would be covered with substrate.
Another thing that can be done is paint the 1618 on the floor, and while it is wet, put a thin layer of sand on the 1618, when it sets up, remove any excess sand. The sand that is embedded in the 1618 is permanent and will not come off.