A few notes here.
Heaters are purely resistive... they play havoc with cheap (and expensive) UPS systems. This is not a problem with a large UPS and a small heater, but a modest heater and a low VA rating will spell trobule.
Secondly, a UPS must output a true sine wave if you wish to use it for any type of inductive load (pumps or ballasts). If it is not true sine wave, device or ups damage is a certainty, fire is a possibility.
A large bank of wet cells hooked to a UPS or suitable inverter will run the system for a day or 2 (small power head, no heater, and an air pump). However batteries only have a life span of 12-18 months, from that point on they degrade rather sharply (even your car battery). at ~$50-100 a pop, this gets expensive.
The money would be better put into a small 7kW generac that runs on natural gas and has an automatic transfer switch. you will spend about $2500 on the whole thing. A UPS big enough to run an air pump and power head for the weekend is going to cost well over $1000 and a few hundred every year or so for cell replacement.
In the real world, UPS systems are to smooth out intermitant power problems (10 seconds or less) and allow the generator time to kick in and come up to revs with clean power (target less than 30 seconds form power failure). The UPS system is designed to keep the systmes up until generator power AND long enough to initiate full safe shutdown if the genset does not come up.
I hope this helps.
