To reference a Koi pearl is similar to a bottomless tank. In a pond top is sealed bottom is open and you vacuum water into the tank and its opening is below the water surface so it will allow fish to swim up and out of the pond to be viewed. The buy able ones come with a feeder that you can place floating pellets into it to get a good view of the fish, however I have not had trouble with getting fish into my experiments of this type of stuff.