Please look at Pet Smart if you have one - they have started carrying these fixtures or you can order one
you want the one w/ the 12% bulb a 24 watt
Arcadia Reptile ProT5 UV-B Kit | reptile Heating & Lighting | PetSmart
or this one a Sunblaster NanoTech Pro T 5
https://hydrobuilder.com/sunblaster-nanotech-t5-ho-fixture-with-6400k-lamp.html you will need to change the bulb out for this a 22" Reptisun 10.0 T5
Zoo Med ReptiSun 10.0 UVB T5 HO
there is another fixture that I cannot find they are sold out its the 24" Zoo Med this one comes w/ a 5.0 bulb you would need to change the bulb out to the 22" Reptisun 10.0 T 5 I posted up above so if you happen to come across that fixture you can get that one- Pet Smart carries the T 8's they are good but for the 50 gallon tank you want the T 5 -- as posted -- when you get the fixture please post back on here so we can help you get it set up for the tank -- the screen is going to determine distance and placement