8 Oz of black colorant could go along ways depending on how you use it. At the ebay page, Polymer products claims "This 8-ounce kit will tint up to a gallon of epoxy resin to an opaque color." If you plan on having several layers of epoxy, you won't need every coat to be opaque. You might want to start off with just a light tint, then get a little darker each coat, so you can see if you missed any spots. There's no point in going opaque every coat, since you won't be able to see any of the previous coats. Going this route, where you gradually step up darkness, you may only use about 2 Oz of colorant per gallon of resin.
I'm drilling holes for bulkheads after I confirm that the tank holds water without leaks. That way I can eliminate one possibility of what is leaking.
I'm drilling holes for bulkheads after I confirm that the tank holds water without leaks. That way I can eliminate one possibility of what is leaking.