Actually since 1 gallon adds 6ppm per 10,000 gallons, you take the size of your pool (25,000 gallons - which is 2.5 times higher than 10,000) so to add 6ppm to your pool you need 2 1/2 gallons (2.5 X 1 gallon).

If you add 5 gallons then you will be adding 12ppm (+ any residual free chlorine left in your pool) which should be about 12.5ppm

The next time you test IF you were at 10ppm, then you would only need to add about 1 3/4 gallons to get back to 13ppm.

Don't just add 5 gallons everytime - you want to keep your chlorine level between 10ppm-15ppm. The algae should die relatively soon and your water will turn a cloudy blue - but keep the chlorine level high until the pool is clear.