Hi there. It is very difficult to ascertain sans visualisation or analysis of the water to state that your tank water can have caused this problem. The itchy rash may be unrelated & will require you to visit a dermatologist
or /& upload photographs here to help you better. Use a topical steroid
lotion such as mometasone
furoate twice/daily for the affected areas & see if it responds in 48 hrs together with systemic fexofenadine
tablets 180mg taken once/day. Cheers & Good luck!