Hello,
It looks like contact dermatitis and allergic reaction. You can apply some calamine lotion at the rash as it will help in soothing the skin. You can take some OTC oral antihistamine medications like Benadryl or Claritin and see if it helps.
For mild-moderate symptoms a weak steroid may be used, whilst more severe cases require a higher-potency steroid. But they are available under prescription and should be taken only after consulting a dermatologist. Also wear cotton clothes and use mild soaps like Dove.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Sounds like they don't do a good job of cleaning that tank. You don't say how long you have had this problem. If it is only a couple of days I would wash well wait a bit. If it has been longer than that go see a dermatologist.