I'm sorry that your little beardie didn't make it.
That's so sad.
It is a good idea to make sure everything is good for this one. A 6 mo old is a good age to start with.
You mentioned that you're using an Exoterra strip fluorescent UVB, so that means that it is the Rept-Glo brand. Can you look on the side of the bulb and see what % of output it produces, like 2.0, 5.0, 8.0 or 10.0? That would be very helpful. Being able to get 3" from it is too close and he should only be able to get a minimum of 6" from it unless it is a Repti-Glo 10.0, then the distance shouldn't be any closer than 10 inches, that's very important.
Using a red basking bulb is not a good idea because it can cause eye issues and it also doesn't help reduce the glare of the
UVB light. A bright white basking bulb is best and it should be right alongside the UVB. A 150 watt bulb seems like an awful lot of wattage for only a 3 ft tank. I would suggest you remove the red bulb and replace it with a 100 watt white standard household lightbulb for now. It's very important that you find out what the temps really are because if the tank is too hot it can cause a heatstroke and rapid dehydration. I suspect the basking temp is a lot hotter than 85 degrees. The only 2 ways to get an accurate reading are with either a temperature gun or a digital thermometer with a long wire & probe end. Here's a picture of the digital that I bought at Walmart for about $12.00, it's the AcuRite brand and you should find it in the area where outdoor plastic thermometers are sold.
You mentioned that you're using reptile sand, if it's calci-sand, it contains calcium & encourages them to lick it causing a build up over time and hardening in their bellies like cement. And if it's vita-sand, it contains vitamins which is also an impaction risk. I suggest you remove it and replace with even paper towels for now then later on replace with either
non-adhesive shelf liner, reptile carpet or slate tiles.
With a 6 month old beardie, you should be dusting his live food 5 days a wk with calcium with D3 but is phosphorus-free with a product like this on 5 days a wk but only 1 feeding a day:
http://www.petmountain.com/product/supplements/105551/rep-cal-ultrafine-calcium-with-vitamin-d3.html
Keeping babies well
hydrated by
bathing 3 to 4 times a wk is a good idea with the vent area (where they poop) right down in the water for about 15 to 20 min. Test the water by the inside of your wrist, should be warm but not hot, be sure to put a washcloth or dishtowel down in the bottom of sink or transparent container to help keep them from sliding around.
I know this is a lot of information but want to make sure everything is perfect for your beardie. If you have any questions, let us know. Oh, and we LOVE to see pictures.