I ordered a T8 5.0 bulb not knowing that I needed a 10.0 I’m new to being a bearded dragon owner now can I still get use from the bulb? Or do I need to return it? Will it be good to use if I put the fixture in the enclosure?
With a T8, I would return it in exchange for a ReptiSun 10.0 or Arcadia Euro Range T8 bulb. Those have optimal output at around 6-8'' above the main basking area. The 5.0 wouldn't really be effective at a safe range so wouldn't be of use. If you move to a T5 then you can get increased effective output if needed. A T5 10.0 is best at around 12-14''. It comes down to the distance you need to cover between the bulb and basking zone.
Moved by moderator from Advanced Discussion to Enclosures.
Might be OK if the tank is small , you mount UNDER the tank lid ( no obstructions / shading effect ), you have a good T8 reflector hood or slip on T8 reflector and the distance from the basking spot to the tube can set up to give about the right level of UVA & UVB flux.
Tube --> basking spot distance for a 5% T8 in a reflector hood is about 7".
You really want 10% or higher... Honestly, as high as you can find. Make sure that you are replacing that bulb every 6 months. It may still turn on, but the UVs demenish with time. You'll notice the difference the first time you do a swap and your new bulb burns holes in you retinas.