I don't believe you can ever have too large of a sump.If you are using CO2 an over sized wet/dry can make keeping Co2 at optimal levels hard but the advantages out weigh the problems of maintaining high Co2 levels worth it it The oversized wet/dry filter and sump really help maintain very good water quality and I don't have any problems growing plants. I use a lot of Najas guadelupensis which grows so fast that nitrate levels are barely detectable and with regular harvesting I am exporting nutrients. I still hold to frequent water changes in the system. I have 4 T-5 48inch NO T-5 tubes so I barely have about 2 watts per gallon for lights but it seems enought for Echinodorus and Crypts as well as the Najas to grow and reproduce so that the plants seem to be getting enough light to make do with the available CO2. The tank is a 75 gal std plus 30 extra gallons in the sump.