I realise that this may sound like a stupid question, but I would like to explore the issue of water changes a bit to try and understand the chemistry of what is happening in my tank. So, as background, I run a 350litre planted tank (pressurised CO2 with inline difuser, 2 big Eheim filters rated at around 900litres per hour (or something) and a couple of big pumps for water circulation, RO water, lots of plants with attractive latin names, a small school of Denison barbs and some of those long silvery fish with the black line that are famously partial to hair algae (Siamese flying foxes or something??)). I've been in the process of immersing myself in the world of planted tanks over the past year and learning as I go with some muted success - I am yet to grow any of the fabled HC, but I think I have successfully conquered algae (although, in response to this brazenly arrogant claim, my tank is probably furring up with toxic green sludge as I write this). I typically change about 20% per week, which is actually quite a hassle given the amount of RO water I need to produce and I have a nagging concern about how much perfectly usable water the RO machine must be discharging. I live in London where the water apparently passes through 7 people between source and the sea, so I am reluctant to use any tap water in my aquarium. I recognise that passing through a human gut doesn't necessarily render water unusable but, nevertheless, I have had trouble keeping fish in the stuff before. So, I am left in a situation where I truly loathe making water changes. Anyway, I was recently enlightened on the subject of nitrates and phosphates by the wise Greg of the Aquatic Design Centre. In response to his teachings, I have been targeting the desired 10:1 ratio and have found that even without any additional EI fertilisers my tank seems to be fairly steady at something approximately within that range. Arguably both could be lower, but if I don't add any minerals, both seem stable over time. Indeed, this week I have not made any water change and both have been pretty steady. Nitrates are probably slightly higher than 12.5mg/l and Phosphates are probably around 1.5mg/l. I am reluctant to place too much faith in the exactness of any of these home test kits: telling whether the water contains 25mg/l of nitrate or 12.5g/l is like trying to explain to your 3 year old son the difference between a mandarin and satsuma. So, this leads me to my questions: why do I need to do any water change at all if the aim is to keep nitrates and phosphates at a safe level, and both seem to be stable at safe levels anyway? If I keep my plants growing strongly with good light and good CO2, add the necessary micro nutrients and prune regularly, is there any need to change the water? What else can be building up in the tank that I don't know about that might need diluted with fresh water? And, finally, if I don't add the macro nutrients in my EI kit am I failing to add some other magic ingredient needed to fend off algae and keep my plants growing? Of course, should I increase the bioload then I'm sure the current stable condition will degenerate, so all of the above is founded on the supposition that my nitrate and phosphate levels remain at a safe level and the correct ratio. Any help would be appreciated and thanks in advance. I fully intend to upload some pictures of my tank as soon as I can capture an image with the barbs sitting vaguely still: they constantly charge about the place and so every photo is covered in these little red streaks. Actually, I note that there is not a tutorial on photography on this site... I would love to contribute myself: maybe one day I will be wise enough to do so.