Sorry I was out all weekend.

For a 7g I would not worry about CO2. Just add a bit of Excel and do weekly water changes. Even a DiY might be too much CO2. You could add a second line (Y-splitter) and just disappate some of the CO2 into the air if you decide to try DiY and get too many bubbles.

For kH try adding 1/8tsp to a 5g bucket and test it before and after. Adjust this to the amount of water you change weekly.

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--->Pictures of my 75g Amazon tank<----