Kind of depends upon what other components you are thinking of using, Debbie. If your native soil is sandy like ours is here, you could go maybe half and half with the compost, but if you are thinking of using potting mix, you should also mix in some extra perlite or coarse chicken grit or something for aeration. Might be a good idea to check the pH of your soil or other components before you mix, too.
Does your stock tank have plenty of drainage? Compost can be very moisture-retentive so you want it to be able to drain freely so that it doesn't get too soggy.
Also, be sure that your compost has completely stopped "working" before you begin to use it. Some people have ordered a truckload of mushroom compost, and when it arrived, it heated up again from being all fluffed up with air in the moving process. If it's still working it will actually use up nitrogen out of the soil which can rob your plants of that necessary nutrient.
