KH In The Aquarium

Low KH is not a problem as long as you are aware that it is low and that you need to keep an eye on your pH. The natural tendency of a tank is to become acidic and with low KH - that is, hardly any carbonate - the acid uses it up quickly making a pH crash possible.

As for the frequency and amount of water changes, it is really up to you. But 7 x 10% changes a week doesn't add up to the same as 1 x 70% water change because each of those 10% changes removes some of the water you added yesterday. And some of the water you added the day before and the day before that. But 10% a day will replace more KH that 25% twice a week.

Mind you, 20% every day would top up the KH better still