THis is an odd question. You can not use an ro unit in the manner that you are asking about. The "top" piece you ask about is the ro membrane. You can not get 1000 gph out of the average unit. They are not designed to move large amounts of water or to be used for heavy dirt trapping.
Lets review. The typical ro unit has 2 or 3 or more modules. THe first is the sediment trap, looks like a tighty wound spool of string. It's purpose is to trap tiny debris. After your water flows thru it, the water goes to the activated carbon block, it's purpose is to perform chemical filtration,mostly to remove chlorine from your tap water. Most units go to the ro membrane next. It will have one water inlet from your carbon block and two water outlets. One outlet is the "clean" water that the membrane processed, the other is the "waste" water, the stuff the membrane rejected that goes down the drain. From here the "clean" water either gose to what ever it was intended for (normally a storage vat) or the processed water goes thru additional treatment like D.I. or membranes for silicate or phosfate removal. Well maitained ro units are good at what they are intended for, trying to modify one for an aquarium filtration unit would not be very benificial. the sediment trap would clog very fast, the carbon block would not last very long. If you continued to use the ro membrane, it would most likley kill your fish because it can remove all elements from your water. If your not going to use it for what it is intended for, your best bet would be to sell it (they are not cheap) and use the money to buy or build a system more suited for aquarium use.
Joel