Is there an air conditioner in the room the tank is in?
With a 90g tank you fall between 2 of the standard models..the 1/10hp chills a tank up to approx 80 g by 10 degrees on average (meaning your tank would get to be 90 degrees and the chiller can cool it down to 80 degrees without overstressing the unit...not that you want it to EVER have that large a temp swing..it's just how they 'rate' chillers). A 1/4 hp chiller can chill a tank up to 180g by 10 degrees easily (hey, never know if you want to upgrade
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There is a 1/6 chiller for a 120g tank but I can't say I have seen them around much for sale.
If the room where the tank is has no A/C and gets rather hot during the summer, you might want to consider the 1/4 over the 1/10.
The issue with plumbing a chiller in-line with your return is you need to be sure that the chiller is getting the amount of gallonage it requires to operate properly. All chillers have a standard 'range' of ghp they require. A 1/10 chiller requires (on average) a miniumum of 500gph to a high of 800gph. a 1/4 chiller a low of 600gph to 1000gph.
As far as circulating in your sump, that isn't necessary. Once the chiller is in operation it should be adjusted so that it turns on when the water is about 2 degrees 'off' from the temp you want it to be at so it isn't shooting a bunch of icy water into your tank if that is your concern.
It is strongly suggested that chillers be run off a controller of some sort so they will turn off if the chiller doesn't stop chilling when it is supposed to.
The downside to chillers is they produce a large amount of heat, so they need to be placed as far away from the tank as reasonably possible. Remember -- it will suffer from 'head-loss and require a larger pump the further away you move it!
Best to research the chiller you are interested in to find out it's requirements for pump size and the amount of clearance it needs to vent itself properly.
Hope that helps
