in a low tech tank very infrequent water changes are recommended on the basis that exchanging water causes fluctuations in co2. i know there is a tom barr piece of work that shows fluctuations in co2 resulting in increased algae growth. (i think this was in a high tech tank experiment??? or is there further evidence as well?) why then do weekly 50% water changes with EI dosing and injected co2 not cause a similar problem with co2 fluctuation every week? i imagine this is partly because with co2 injection and high light the rate of co2 turnover is much higher and so the effects of the fluctuation are shortlived. not wishing to question established wisdom, but trying to understand things a bit better. * if adopting a PMDD approach then large volume weekly water changes are less important as lower doses of nutrients are used. if water changes encourage algae via co2 fluctuations would it be better to do them less often if using PMDD? * what about new set ups where ammonia levels may be higher - often waters changes more than once a week are suggested, presumably this is because experience shows that raised ammonia with poorly established plants is more likely to promote algae than fluctuating co2? thanks.