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My basking temp is really high like around 40 degrees Celcius how can i lower it to the right level?

All you can really do is mess around with the light until you get the proper temp. I think we all fiddle with our basking lights for ages before getting it just right.

You have to move the light farther away to get a lower temp and if you can't do that maybe get a slightly lower wattage.

I had to try a couple of different wattage bulbs before I found the right one. My basking temps were as high as 110 degrees. My RESs didn't like it and would never bask so I bought a less wattage bulb and moved it further away from the basking area. Now the basking temp is 96-97 and they are basking all the time.

Thanx for the quick reply i will try both of those. He basks but i still think its to hot for him. one more question if you don't mind. I have a screen on top of my tank and its a tight wound one not big gaps is this ok?

A screen with a tight screen blocks out a lot UVB. Are you using a UVB lamp? Also, get a thermometer to properly measure temperatures.