I bought 2 young male Rats last week, and 1 keeps sneezing constantly!!?

they live in a 1.5 metre x 0.5metre glass aquarium with wire lid for good aeration, i took the advice from pets-at-home staff and use Bio Cat Litter as the base, and pets-at-home paper bedding, One rat is fine and hardly sneezes but the other sneezes/coughs constantly. should i change the bedding or base litter??