We recently installed a new water tank as old one had a leak.
Only problem is hot water is a disgusting brown.
We have drained tank several times but so far no joy.
Any suggestions?

Since this is a brand new water heater the "brown" water is not coming from the heater. It is most likely coming from your old galvanized pipes. Do this: Go to the most distant faucet in your house. Open both hot and cold water and let it run for 30 minutes. See if it helped. Also, make sure you have aluminum or brass nipples on top of water heater. Back to you. Milo