I need help working out how many watts I will need for a low light tank. I'm having anubias, ferns, and crypts. The tank is 48in x 24in x24in and I know the watt/gallon rule doesn't work with tanks over 100g. Thinking about usingT5HO bulbs.
A 48" long tank means you can use 4 foot shop lights as a cheap way of lighting the tank. For a 120 gallon tank I would want about 6 40 watt T12 or 32 watt T8 bulbs. The 24 inch width of the top would allow plenty of room to fit that many bulbs in. And, that light would be pretty uniform over the entire tank, which can't be said of most "high tech" light options. Cheap bulbs too.
If you want to go for high light intensity, using T5 light kits would probably work much better.
I'm using two 4' T-8 shop lights (10,000K and 8500K) over a 125g Tanganyikan Community tank and it is giving me fits with algae...hair, BBA, staghorn, etc. The tank is sparsely planted and I don't dose it (about 40 juvenile fish to supply nitrogen) so you may do a bit better than I am doing if you plant the tank densely and add some type of fertilizer
The tank has algae issues with an 8 hour photoperiod but I have ran it the past two weeks on a 6 hour photoperiod and the algae definitely declined. Unfortunately, plant health also started to decline recently. I think four T-8 tubes are a bit too much for a sparsely planted 125g tank. In my opinion, T-5's are overkill for a low tech or fish only tank (meaning mainly fish with accent plants). It is just too much light in my opinion.