Why cant we do 100% water changes all the time?
 

Why can't we do it even if we only have small tanks?

Because you will mess up the bacteria population in your tank. So basically you be cycling your tank all the time.

What does the bacteria do?

In freshwater the bacteria will break down ammonia into nitrite then into nitrate. After the tank cycles you should have a ammonia and nitrite reading of 0 and a nitrate reading as low as you can get it. When you do partial water changes you the owner remove built up nitrate. If you remove all water you remove this cycle and are forced to recycle. High ammonia levels are deadly for fish as nitrites are deadly but not as deadly. Nitrates can be deadly up to a point. Fish cn tolarate nitrates more then ammonia and nitites.

You can do 100% water changes...as long as the water quality between the old water and the new water isn't enough to shock the fish...

...the nitrifying bacteria don't live in the water column...

...they live on aerobic surfaces on the media in the filter...on the gravel...glass, decorations...

...those shouldn't be changed 100%...

Well regardless of the cycle and the bacteria, moving your fish to another container, draining the water, adding new water, treating the water, trying to match the old as close as possible especially when it comes to GH, KH and pH is simply going to stress your fish out so bad that I wouldn't doubt it if they didn't live long.

And yes, 100% water changes will cause the bacteria to die if they are left uncovered for any amount of time, starting in as little as 5 minutes and maybe faster if the tank can dry out at all. Even a 50% water change can cause bacteria to die if the water level is left low enough for 5 minutes or more. The bacteria simply dry out and die in short order. I had one of my AC HOB stop running for less than 30 minutes and it caused a major mini cycle.

As long as everything stays wet and has circulation to bring oxygen to the bacteria you are ok but with 100% water changes you take away all the oxygen from the lower bacteria in the water and take away the water that will kill the upper bacteria if the substrate dries out at all. This doesn't even account for the bacteria lost off all the ornaments, the sides of the tank, the intakes of the filters and everything else in the tank. Add a really hot day and the bacteria die even faster.

Not a good idea to do 100% water changes unless you are talking about having the water ready, GH, KH and pH matched to the tank water and temperature as well as dechlorinated and ready to go right back into the tank. If you can do this in less than 5 minutes, I say have at it. 100% water changes done the right way are better than 50%. I for one will stick to my 50% water changes so the filter can keep running and I never have to worry about any more bacteria dying off than needed.

The reason I know the bacteria die fast. I completely emptied a tank once. Removed the fish and put them in another container. Gravel vacced as much as possible to clean the tank and even tilted the tank to get as much water out as possible. Refilled the tank and had everything up and running in less than 30 minutes. The results were a huge ammonia spike and having to recycle the tank for 3 weeks before the ammonia and nitrites went back to 0.

Yes I did it in less than 30 minutes, 26 minutes to be exact because I timed it. I figured if I did it in less than 30 minutes I wouldn't have to worry about a cycle. So much for that theory.

...yes...the media has to stay wet...no doubt there...

If you really want to change as much water as possible every time, change as much as you can leaving the filter running, and refill, and do it 3-4 times once a week. If you can change 60% each time, 4 times, then you actually change close to 95% without chancing a major bacteria die off. 75% twice gives you 88.5% total change, 3 times give you almost 93.25% overall change. (This is assuming my math is correct, 8) )

Thanks guys. I got it now