Wow that is a lot of tangs for a 60 gallon tank. You can not possibly put that much food in there, your nitrates will soar! I would get a bigger home for them and fast, with that many tangs in one tank you should have at least a 180 or bigger. I am suprised they have made it this long. I would sell some of those tangs off before you kill them if you can not upgrade your tank. For the food, you should only feed them a very small amount of food twice a day. I would put a quarter cube in the am and a quarte cube in the evening. OR, I would stop using cubed food all together and feed pellets and seaweed. But again, not such big amount.
120 Gallon Reef
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