Cover the aquarium with plastic good and tight. The chemicals take around 1-2 hours to dry. The chemicals are not a true gas so the amount in the air is very minimal. Any experienced tech knows that spraying near a tank or above it is a no no unless you absolutely have too.. Ask the tech to inspect the aquarium, the stand and surrounding areas really well. If he finds them, he should do a crack and crevice treatment which is very controlled and direct spraying. I have found bedbugs around aquariums many times, they are attracted to co2 and heat, if you have a co2 system you could have bedbugs near the tank. You'll be fine. 8+ years of pest control experience and 100's of bedbug jobs.
PS: never in 8+ years have I seen a customer's fish die from a bedbug treatment.