I have done this on a 150 for a friend before. We got rubbermaids and siphoned 75% of water out. Drilled the hole with a Dewalt cordless drill. Then siliconed a premade glass box on and supported it with precut glass to the right height. Then filled the tank back up to the glass box so that the silicone could cure overnight, and the tank was just running on powerheads overnight. The next day it was plumbed and I don't think there were any losses of any sort. This was over 5 years ago and the tank is still running just fine. Hope this helps.
Des
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