[Myka]

Imo that return pump is insufficient. Ideal flow through a sump is 6.1 times per hour. This number is calculated to allowed 99% of the aquarium volume to pass through the sump every 24 hours. Since a protein skimmer can only process a certain amount of water, this 6x turnover provides the protein skimmer with an ideal amount of "old" water to filter. Keeping this in mind, the ideal turnover for a 90 gallon aquarium with about 20 gallons in the sump is about 670 gph (after head loss).

Is your return line drilled or does it run up over the edge of the tank? If it is drilled for a 3/4" bulkhead then increasing the plumbing without increasing the bulkhead would provide minimal improvement. Although with an appropriately sized return pump for a 90 gallon display, a 1" return line would be best. I would suggest you look at replacing the return pump. A Quiet One 4000 pump would be quite a good fit for your system, and is a popular pump at a reasonable price point. An Ocean Runner 3500 would be another good option although I find my Ocean Runner to have a slightly louder hum than my Quiet One pump.

As far as powerhead flow goes, it depends what kind of corals you want to keep. When calculating turnover, just use the volume of the display tank (don't include the sump). For softies and LPS I find 10-30 times turnover is ideal, and for SPS corals 50-100 times turnover is ideal.