the greater the amount of organics in the water the stronger acidic buffers will become. on a closed system eventually the alkalline buffers will be overcome by the acidic buffers. depending on the ppm of alkalinity will determine how quickly these buffers will be broken down. if you want to break down the alkaline buffers and do not want to use acid, you can put acidic peat (pH of 4) in the system. for many veggies a pH of 5.4 to 6.3 is preferred. as badflash said there are plants that are cultivated to withstand higher pH's. the goldfish will not have a problem with any pH from 5-8.5. it is the plants ability to take up nutrients that are bound up as the pH becomes more alkaline (or base).