how many eheim filters should i use on a 80 gal

I believe that particular set up is designed for tanks up to 160 gallons... so the answer to your question would be, one.

It really depends... what will you stock the tank with, and how many fish? Also, will the tank be planted?

Jim

1 should be plenty

I run a 90 with 2028 and it more than does the job.
The tank is heavily planted, but also has a very high bioload.

What will help a great deal is using some sort of prefilter on the intake- whether you use the Eheim prefilter or a Filter Max foam one. A prefilter is easily removed for frequent rinsing- this keeps the bulk of the solid organic waste from ever getting to your "real" filter.

Using a prefilter I can go ±6 weeks without breaking down the filter. That 2028 is heavy when filled with water and sitting dead on the floor in a cramped position!

It is a great filter choice- very easy to use and totally silent. Totally!

Edit: brand name prefilter

It really depends on what's in the tank. A single 2028 wouldn't be enough for an unplanted, moderately- to heavily-stocked cichlid tank, for example. For less messy fish (e.g., a school of tetras), it would be plenty.

Agree with the ideas about prefilters. I think the Filter Max IIIs are great.

Jim