That applies depending on what else is there. If there is nothing, the tank is completely empty, then a 5 gallon holds 5 gallons. With rocks, a heater, a filter and plants water is displaced, therefore yes, there is less water. Which is why it is so iffy with adding frogs to your tank. For instance my 20 gallon community tank (now successfully killing the ich parasite :) ) probably holds 18 gallons because of rocks and ornaments.
It isn't really "half" that it holds... always depends on what you have in there :)
Haha wow... btw good job on a good sized tank for the goldies. And secondly, mine is the smallest I could find.. And I got it from walmart. about 12 dollars, and does okay for smaller spaces... Do you have gravel in the betta's tank, or larger rocks? even a simple tube works if you have rocks and not gravel. Otherwise gravel gets stuck in the tube ><
Breed for the breed, not for the money; the words any REAL breeder would understand.
You must know nothing in order to know all, for all is nothing and nothing is for all.