Since someone necro'ed a 7 y/o thread, a couple of things here...
In my opinion...
First of all, the algae tanks are a colossal waste of space. 24 sq ft per person is A LOT of space in any bunker/shelter. Unless you're expecting to be repeatedly hit with chemical weapons over a long period of time, it's not necessary to maintain a sealed shelter atmosphere over the long term. You certainly don't need that for fallout. 24 hours is generally all you'd need. Even that is a bit overkill because the majority of weapon fallout does not "float" through the air at ground level, doesn't flow into structures, etc. All of that is Hollywood nonsense.
In the most basic sense, changing filters requires shutdown of the air exchange system, sealing the inlet, carefully changing out and disposing of the used filter. Again, that would only be required if your location were being hit multiple times over the course of days with chemical agents like Sarin, Anthrax, VX, etc. Other than agents which are spread via a medium like Anthrax spores, biologicals (even airborne agents) travel from person to person. They don't float through the air long range like a gas. On the nuke side, it would only be required longer than 24 hours if you lived next to a stricken nuclear power plant or spent fuel pool fire and the winds NEVER shifted. Even if you lived due east of one, the winds don't blow out of the west 24/7/365.