I'd agree that if you could avoid the bombs all together you'd probably be much better off.
The following is how you would need to secure your tank for a pesticide bomb AND why you shouldn't use them if at all possible.
If thats not an option then you will have to be sure to have a 100% air tight enclosure around the tank. Which will be nearly impossible when you consider that 100% tight means no oxygen for the tank. So inorder to get oxygen to the tank you will have to duct some air into the enclosure. You will need some sort of large diameter tubing, like a clothes dryer vent hose and a fan that will push air through that. You'll need enough to get fresh air from well outside the area that is being bombed, likely outside if at all possible. You would then also need a vent on the enclosure to to then vent the air out of the enclosure slightly slower than you are pushing air into the enclosure. What this does is creates a possitive air preasure environment in the enclosure which means any leaks in the seals of the elcosure will leak fresh air out rather than let peticide laden air in. This would have to run non stop @ 100% for the full time the bomb is being used, and I would also suggest letting it remain in place for a day or two afterwards. As you begin to return to the area some particles that have settles on furniture, the floor, etc will get kicked up into the air. It's not enough that it would harm you, but it can easily affect your fish tank. Now, because this entire enclosure is air tight and will be in place for at least 2-3 days... that means you fish don't get fed, i don't really know how to get around that issue... perhaps sprinling flake food in front of the intake fan? lol... as you can see... this is NOT a reasonable and certinly not an easy solution. avoid the bombs if at all possible!