Since we are going into the cool season, you might be better off starting up with goldfish (or perhaps do your fishless cycling first while you monitor your water temperatures in the greenhouse through the winter) then once you are fishlessly cycled up you could decide if you still want tilapia (remember that tilapia don't really eat or grow much while the water is below 70 F and if the water gets down to 50 F you will probably have dead tilapia since 55 F is near their minimum temperature depending on particular species) or if goldfish would be better or another type of fish.