Turtles/Russian Tortoise neck discoloration
I have a Russian tortoise that I purchased from Petco about two months ago. I brought him to the vet within a week, who told me that Gamera was six months old and had his stool tested for parasites (he was fine). We feed him a variety of greens, have a shallow water bowl, bathe him twice a week, and let him outside daily for about 20 minutes to get some natural sunlight, otherwise we do have a zilla UVB light, which we keep on for 12 hours each day, and 100 watt basking light over a 60 gallon terranium. Typically, it's about 85-95 under the basking lamp, and around 65-70 on the cool side. We use a mixture of topsoil and playsand for substrate, with some coconut coir under his hidey. He's very active and has a great appetite, but I'm just worried about his neck, which has a bump that looks like the rest of his skin but raised, with a large gray mark around it that almost looks like bruised skin.
I'm afraid your vet isn't very familiar with tortoises. Petco Russian tortoises are all wild caught adults, generally in the 4" range and probably around 350-400g. A six month old juvenile is much, much smaller (generally in the 50g range) and anyone who knows tortoises could not mistake one for the other. It's good that the fecal check was negative, however, since the importation process is extremely stressful and it's not uncommon for new imports to be loaded with parasites.
I'm not certain what the bump is. It almost looks like a bit of shed skin that has bunched up. You can see if it changes or loosens at all with soaking. Is the darkened area something that just developed recently? Other possibilities are that he scraped it on something, it's residual from a prior nip, possibly some kind of warty growth, etc. Tortoises aren't really prone to developing tumors. In any case, I don't think it's something to worry about too much, but keep an eye on it for any changes (reddening, oozing, odor, etc.) and also watch for any overall changes in him such as lack of appetite or lethargy.
Your habitat sounds pretty good. I would consider building a tortoise table at some point; glass tanks aren't ideal for tortoises, although a very large tank can work if it provides good air circulation. Make sure the substrate stays slightly damp to help avoid dehydration, which can be a significant issue with tortoises kept indoors with basking lights. Keep an eye on his urates--they should be watery to soft and creamy. If they become gritty, it's a sign of dehydration.
The Zilla bulbs are unfortunately not a good source of UVB. If you can get him outside for several hours a day--and I highly recommend building an outdoor pen for spring, summer, and fall--you don't need to provide UVB indoors. Otherwise, I would get a ZooMed Powersun bulb which will provide both heat and a good amount of UVB. The bulb needs to be replaced yearly, or the equivalent (that is, if he's outside six months of the year, you'd need to replace it after two years).
Let me know if the bump changes in any way, or if any other problems develop, and I'll do further research for you, but at this point I don't think you need to worry.