Not sure if it will work in your application, but you could hang a 2-tube shoplite over the tank...assuming you have glass tops or can go open-top. The shoplite is 48" long, but you can use the 'extra' light to light up a small QT or growout tank too. With 2 32W T8 bulbs it works out to ~ 2W/gallon, considering the light 'spillage'. I have a shoplite over an open-top 20Long; I have no problem with growing amazon swords, or any other low-light plant-I also have 6 mangroves doing nicely in there.
+ you can hang the light high enough that access is easy.