One of the easiest ways to make a fish tank is to hook a trash can up to a rain barrel. This gives you an extra 50 gallons of water to use in the garden, a place to drop your garden pests and it helps get rid of mosquitoes.
To do this, simply run a hose off of your rainbarrel into a trash can. As your rainbarrel overflows, it will refresh the water in your trash can and eliminate the need for an aerator or pump to keep the water from going stagnant.
An important step when building a trash can fish tank is to drill small holes all around the rim of the trash can, a few inches down from the top. This will strain the water as it leaks out of the holes, rather than gushing over the top – taking your fish with it. I learned this after finding my goldfish flopping around in the yard after a heavy storm.