you have some issues with husbandry. The bedding should be organic soil and play sand mixture and 6-8" where the tortoise can dig. This is very important. The next issue is hoousing. The should never be kept in a fish tank. A large rubber made bin or a tortoise table is much better. This is a tortoise table that should a good set up: http://www.pets4homes.co.uk/images/classifieds/2013/05/11/300984/large/hermann-tortoise-with-complete-set-up-inc-table-518e01218fee9.JPG The tank restricts the air flow which can cause respiratory issues. so it is much better to use the tortoise table.
The lighting is the next issue. The light you described does not sound like a UVB light. The UVB lights give off te Artificial sunlight that works with calcium to metabolize the ccalcium in the body into Vitamin D3 and prevent Metabolic Bone Disease. I suggest using a Reptisun 10.0 linear tube. If the lighting is not correct as well as the husbandry, the tortoises will loose appetite and and become lethargic. The temps should be : Daytime Air Temperature: 70-90°
Nighttime Air Temperature: 60-65° Basking Temperature: 90-95° and humidity Humidity: 30-50%.
The Calcium should be plain calcium no D3 or Phosphrus. This is very important. You can add cuttle bones for birds and scatter them in the enclosure to offer a natural calcium. For diet natural weeds like dandelion is great, but if you feed store veggies you can use endive, escarole, shredded carrot, kale, opuntia, occasional romaine, and occasional pumpkin.
As far as the Mites, you need to clean the enclosure and dispose of the bedding. Mites can kill a tortoise. I suggest that you give them a soak in a Diluted Betadine solution( Provodine Iodine) and water. While in there, use a soft tooth brush and clean shell scales and body. you may have to repeat this a few times.
I think if you clear the mites and fix the habitat as explained, they should respond and start eating.