Do these fit in a Jet 750 tank?

Yes they will fit in a Jet 750. If you have anymore questions feel free to let us know.

how long can you store sealed containers?

I have noticed no degradation in the tablets in the 6 months or so that I have had the containers. That they stay sealed until needed is a pig advantage.

How long does 1 tablet last in an aerobic system?

I use these in a drinking water 1000 gallon reservoir system and 1 tablet for treating drinking water is good for about a month, I imagingine the rate of dissolve would be similar in any water based system of similar volume of holding and usage.

Are these 3" tablets?

There are two sizes of tablets manufactured for aerobic treatment systems by four firms; 2-1/2" or 2-5/8"; 2-1/2" tablets are the premium tablets on the market. Septicfit is a 2-1/2" tablet specifically designed for aerobic treatment systems. The common 3" tablets found at pool supplies are the wrong chemical and should not be used in aerobic treatment systems.