wouldnt not doing water changes be better?

[Hans]

wouldt you be losing a lot of nitrate? it seems it would be better to leave all that poop in the gravel for fertilizer, and the nitrate in the water will be less if you do a big water change right? what am i missing?

[IndianaSam]

If ammonia was the only thing that fish expel, then that would be a solid argument.

However, they also expel hormones in their waste. These hormones actually inhibit growth when they're at high enough levels. It's sorta a natural way to ensure that they don't outgrow their environment. At least that's what I was told many years ago.

This doesn't mean you can put an oscar in a 5gal aquarium and be fine (I've read lots of your post and know that you know this, that statement is mostly for the newbies who may read this). Like all things it needs to be in moderation. Too much / any retardation of growth is detrimental (as you also know).

[RTR]

High nitrates in the water will promote algae more than plants (10ppm nitrate is fine for vascular plants).

If it were nitrate alone in the water, you might justify not changing much. But how about colorants (water yellows with time and pollution), all the myriad DOC (dissolved organic compounds/dissloved organic carbons - pheromones, hormones, phenols, aldehydes, etc.), loss of KH due to normal nitrification leading to downward drift in pH, increase in TDS (total dissolved solids) due to evapotation and feeding ...et cetera ad infinitum.

IMHO, not changing water is just about the worst thing you can do to a tank.

Water is cheap, change it often.

[Richer]

Well said RTR.

Another thing to consider. In a high-light, CO2 injected, supplemented tank, doing large water changes can help "reset" nutrient levels in a tank. No matter how precise we are with our dosing, its nearly impossible to get it right... some things will most likely be present in excess

-Richer