Well I don't think it's a Respiratory Infection at all, but rather some husbandry issues, first improper temperature zones, and then inadequate
UVB light. You already know that your 5.0 UVB tube is only a 5% UVB tube, but since it is the stronger T5HO, you can still use it and make it work, you just have to mount it correctly. If you have the 5.0 UVB tube sitting on top of a mesh lid to the tank, then he's not getting much
UVB light at all, and much less UVA light than he needs as well. When using a UVB tube that is only putting out 5%
UVB light and only 30% UVA light (as opposed to 10% UVB and 60% UVA like the 10.0 tubes do), you have to poke holes in the mesh lid over the Hot Side of the tank, and then use either long zip ties, wire, twine, shoelaces, etc. to strap the entire tube fixture to the underside of the mesh lid over the Hot Side of the tank. Then you must ensure that his Basking Spot/Platform is within at least 6-7" of the now unobstructed tube, which is about half as much closer than the 10.0 T5HO needs to be to the basking spot/platform (10.0 only needs to be within about 11"). Also, you have to replace the 5.0 T5HO tube once every 6 months, as opposed to the 10.0 T5HO which only needs replaced once a year. So if it isn't already mounted inside the tank and underneath the mesh lid, and within about 7" or so to his basking spot/platform, then you need to get this done asap. Also, be sure that in addition to the UVB tube being over the Hot Side of the tank, you need to make it so that it is right alongside his bright-white colored Basking Bulb, so that both of the lights are over the Hot Side of the tank and basically "sharing" the top of the Hot Side of the tank. Then position his Basking Spot/Platform directly underneath both of the lights within the Hot Side of the tank, so that he gets both lights at the same time while basking. This is replicating natural sunlight as closely as possible. And I highly recommend that when that 5.0 T5HO Reptisun tube hits 6 months old that you replace it with the 10.0 T5HO tube, which you can then sit on top of the mesh lid as long as it's within at least 11" of the basking spot/platform, and it will only need replaced once a year. the 22" Reptisun 10.0 T5HO UVB tube only costs around $24 on Amazon.com.
Now you've got a real temperature issue inside his tank...Those lines on his belly are not "stress lines", that's a common mistake or "myth" that developed on the internet and stuck. They are not at all an indication of stress, they are just markings on their bellies, just like the markings on the rest of the bodies. And the reason they darken, as well as the reason the rest of his body is darkening, is because he's trying to attract more light/heat, which is completely normal for them to do, and they will do when they are underneath their lights, because they know they are under lights and they want to attract as much heat/light as they can. That's also the reason that they lighten/brighten up when you take them out of their tanks, because they are no longer underneath lights to attract, so they don't need to make themselves darker anymore. So this is absolutely normal and not anything to worry about at all.
However, your temps are far too low. I don't know how old your dragon is, but your Basking Spot Surface Temperature (which must be measured by either a $10 Digital Probe Thermometer or a Temperature Gun, NOT a stick-on thermometer), needs to be between 105-110 degrees F if he is a baby or juvenile/sub-adult that is a year to a year and a half old or younger. If he's an adult that is at least a year and a half old, then it needs to be between 100-105 degrees F. Then the Hot Side Ambient (air) Temperature, which is the air temperature surrounding the basking spot/platform that is located within the Hot Side of the tank, needs to be between 88-93 degrees. Then the opposite Cool Side Ambient (air) temperature needs to be between 75-80 degrees. So once you get your UVB tube mounted inside the tank/under the mesh lid and within at least 7" of his basking spot, then please re-measure all 3 temps, starting with his Basking Spot Surface Temp, and then the Hot Side Ambient (air) Temp. Once you get them within the correct ranges, then you can worry about getting the Cool Side Ambient (air) Temp up to 75 degrees. Depending on the size of your tank, you may need to add a second, much lower wattage bright white colored basking bulb in either a clamp lamp or a dome lamp over the Cool Side of the tank. Usually a 30-40 watt will do it.
Again, I don't know how old he is, but if he's under a year to a year and a half old, he probably won't eat many, if any greens or veggies. Some eat none. You need to offer fresh greens/veggies every single day, so he'll get used to them, but if he's still a baby/juvenile/sub-adult he doesn't need the greens/veggies, he needs a ton of live insects every single day because they do about 90% of their growth and development during their first year of life. That's why they typically don't eat a lot ir any greens or veggies during their first year of life. And even some adults that are 2 years or older refuse to eat a lot of greens or veggies, some of them just won't do it. So if that's the case, then you need to be giving him at least 2 live insect feeding sessions every single day, that last between 10-15 minutes, where he's allowed to eat as many live insects as he wants in that time period. You also need to be dusting all of the insects from 1 of his daily live insect feeding sessions in calcium powder 5 days a week, and in a reptile multivitamin 3 days a week, and this is for the rest of his life.
As far as "
drinking water", Bearded Dragons are desert reptiles from the Australian Desert, which contains little to no standing water. As a result, Bearded Dragons have no concept of "standing water", and in the wild they get most all of their
hydration from the live insects and vegetation that they eat. This instinctual behavior continues in dragons bred in captivity, and as a result most dragons will not drink water from a bowl or a
bath, and they shouldn't need to if you're feeding him a proper diet of tons of live insects and offering a fresh salad every day. If you feed him this adequate diet of live insects he will get most all of the
hydration he needs from his food. You can drip water on his snout for him to lick off several times a day if he likes it and licks it off, and you think he needs extra
hydration, that's fine.
That being said, the only time Bearded Dragons need a
bath is if they are dirty. The absorb no
hydration through their vents or their skin/scales, and so soaking them or
bathing them will not give them any
hydration at all, unless your dragon is one who actively drinks the
bath water through his mouth, which it sounds like he doesn't, as most don't. So baths typically only serve to stress them out and are totally unnecessary unless they are dirty. Doing a
bath once a week for cleanliness purposes is fine and is more than enough, but don't expect it to
hydrate him, this is just another internet myth that physiologically makes no sense and just isn't true...
