they will work but not recommended --- they are a weak bulb -- if you want to keep the fixtures and change out the bulbs for now I would do that -- Reptisun 10.0 T 8 they are a better bulb - the only thing is they need to be in the tank regardless of the screen hole type and directly above the basking decor 6-8 inches -- T 8's are designed for lower height tanks and small tanks like a 20 gallon --- I would look at getting a 24" fixture like a Zoo Med or Arcadia Pro T 5 w/ a 12% bulb - they would be better for your tank size --- then depending on your screen it has options for placement --- and please NO clear plastic cover on the Zilla fixtures take it off pleaseI have a 75 gallon tank that is 49” long. I have 2 18” Zilla slimlines. Will these provide enough UBV with T8 50 UVB desert bulbs? Trying not to buy new fixtures if I can use what I currently have. thanks