use for old pump

[Fish]

I think that the pump would work on your 90. If you are happy with the pump it would be a shame to get rid of it. Are you using a sump?
It might be too big for sump return (unless you have a big sump). You could use it on a closed loop and split the return line into 4 (or more) lines so that the velocity from each isn;t too much. Then just use a small pump for your sump return. Or you might be able to use the pump as a sump return and tee a line off to run your skimmer as well...
With a 90gal tank I would be shotting for around 270gph anyways so... I definitely think you could make it work.

- Chad