Most plant enthusiasts want 'free CO2' in their tanks so that non obligate aquatic plants can grow well under water. We blow CO2 into the water to feed the plants and try not to gas the fish. High buffering at pH 8 ish makes this difficult to achieve. Lower buffering at pH 8 or lower enables more CO2 to be 'free' especially at pH 6.8 and lower and lower buffering enables this to be achieved. This buffering is typically expressed as bicarbonate buffering but in practice is the sum of a thousand and one different ions. Eg the aqueous sulphate ion has a pH of about 7.4. The planty folk also pour dihydrgen phoshates into their tanks which will dissociate into both mono and di salts and form a strong pH buffer. Rain water and purer has little buffering capacity but the water in our little glass boxes has.