Not trying to sound rude but are you sure on your measurements and volumes for this puppy. 12'x5'x6' = 2696.84gallons, not 4000g or are you allowing for overall volume including filter? Also I couldn't help noticing you are 3 besser blocks wide internally. That would work out to 4' if they are standard size blocks as they look to be. There is a great calculator up top by pressing the conversions tab. Again, not trying to sound rude just wanted to clarify and I look forward to seeing this progress.