Thanks Colin-- Ive been keeping fish for 3 decades. Some of the most sensitive, wild species out there available to hobbyists. I know why fish need water changes ( I realize it is more than nitrates we are speaking of here) and I disagree with the necessity for one size fits all assessment of weekly water changes-- huge ones every week. There are many factors and scenarios that would mitigate that necessity: plants, uv sterilizers, density of live-stock, to name a few.
I am aware of bio-load and bacterial load. Thats why two of my tanks that are heavily stocked have top-shelf UV systems.
But, my 6 juvenile fish in my 30 gallon- who happen to be f4 Gary Lange stock- the amount and frequency is more than adequate. Lets be reasonable here- we need to think contextually and not just repeat a general "one-size fits all" rule to fish in the aquarium. .
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I am not trying to be argumentative here, but there is some misunderstanding. It is certainly true that live plants, stocking density, etc will impact or have consequences or be variable factors on the biological system, but none of these can replace water changes.
Fish in their natural habitat are in "fresh" water with every respiration of their gills. This is not even remotely possible in an aquarium unless you have flow-through water which I doubt anyone on this forum has.
Live plants benefit, no doubt of that. But I recall one assessment that if you had six black neon tetras in a 55 gallon well planted aquarium, you would have the ratio of plants/fish that would negate water changes. Plants cannot compensate for lack of water changes.
UV is actually not beneficial in a freshwater system. For one thing, unless every drop of water in the aquarium passes through the UV before it returns to the aquarium--i.e., you would need the UV between two tanks with water passing from one through the UV to another--it is not going to do much. And this still does not replenish the water as "fresh" but merely makes it less beneficial in a sense.
There are substances accumulating in the water from fish that cannot be removed except by a water change. Fish release pheromones that others in the species read, and allomones that are read by other species. These accumulate quite densely in an aquarium, and they cause stress to fish when they are negative. The only way to remove them is with substantial frequent partial water changes.
The other point to keep in mind is that the more water you change the healthier the fish will be, always. Jack Wattley in his monthly discus column in TFH frequently mentioned water changes, and noted that some discus breeders change 95% of the water in the fry tanks two and even three times every day. This allows them to have more fry per tank, and the fry develop faster and are healthier. The point of my mentioning this is not to suggest overstocking tanks, but to illustrate the incredible benefit of water changes. Imagine providing these benefits to all one's fish? Less stressed fish means healthier fish, since stress is the direct cause of 95% of fish disease in an aquarium.
Fish do not become sick from clean water, only when it is not changed. And a 70% change once a week is far more beneficial than 10% changed every day, even though in the end the same "volume" has been changed over seven days. Regular substantial water changes are the most important and frankly crucial maintenance an aquarist can do for his fish.