I am thinking about putting 2 ..20 gal tanks in the back corners of my boat.

My Ballast tank is formed in the hull and holds 30 gals. We fill it with a foot pedal and takes 31 seconds to fill. The pedal lowers a pick up at the transom. There is no gage. When it is full a vent in the deck shoots water in the air. See the picture and you will see water shooting up on the deck. To drain it we have two large Ruel pumps that drain it in 36 seconds.

We only use it very rough conditions and add additional weights to the front which are fixed. They are stainless tanks filled with lead shot and bolted in.