Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I have run UGF systems on tanks up to about 45 gallons.
It takes a pretty strong flow of water to run this large a grid. If you are trying to keep the flow a bit less you might get 2 small power heads, and put one on the top of each tube, one about 1/4-1/3 of the way from the left, the other a similar distance from the right. If there are no holes in these locations, then go ahead and try one PH in each corner. For a 40 gallon tank, you might try 2 PH rated for 100 gph or a bit more.
Make sure you keep the gravel clean. These systems will grow a good bed of bacteria, but they do need a high oxygen flow.