Most fish rise and sink in the water the same way a helium-filled balloon or a hot air balloon rises and sinks in the air. To see how this works, you need to understand the various forces at work in the air and underwater. While these environments seem very different to us, water ...
pump your Balloon, hold the mid part of the Balloon, then open the top to cover the bottle and turn the salt or Sand into the Balloon, eliminate the air and tie the opening. Quite funny, but it looks like fun.