Ok cool. And by the same, do you mean the two 18w, or the four 18w that I've been considering adding?I've got the same lighting over my 29g and they run about 8-9hrs/day.
I think I have done maybe 5 in the past.. but it really isn't ideal.
It is really going to depend more on the type of plants you are keeping, and if you are adding anything extra to the aquarium for the plants.
But, personally, I have used 2 T5HO bulbs (39W) over a 36g bowfront aquarium (so same width and I think height) without any issues, nothing was added to the aquarium (ferts or CO2) and maybe a 9 hour photo period.but the load of fish was pretty small so no nitrates to write home about.
The answer is also going to depend on the fixture, used, especially with florescent lights.. the reflector can have a huge effect on the amount of light getting into the tank.
Honestly, my personal preference would be to go with an LED light, you will likely be able to better control the color of the light (CRI or temperature, whichever metric you prefer) and dim the light if needed too. It is going to be preferable to stop algae growing by either reducing the amount of light or adding nutrients/CO2 to the aquarium rather than shortening the photoperiod to 6 hours... Your plants will be starting to suffer at that point.
Mine is a twin tube (two 18w lamps) Odyssea T5HO I bought "open box" off ebay some years ago for cheap. I've been using odyssea or other no-name mail order 6500K color-temp range 18w replacement tubes that I mail order. I try to swap at least one tube at a time every 6-8 months, but I loose track on that.Ok cool. And by the same, do you mean the two 18w, or the four 18w that I've been considering adding?