It is not a good idea to use lemon juice to lower the pH of your aquarium water. Because lemon juice is very acidic, with a pH of around 2, this can cause too sudden of a change. On the pH scale, lemon juice sits only 1 point away from corrosive stomach acid and 2 points away from battery acid. It would be better to use natural solutions that sit only 1 or 2 points away from neutral on the scale.