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hi, i came up with an Idea, I own 2 red eared slider about a few month old, less than 4 inches in a 30 gallons aquarium.
I was wondering if its a good thing to do a tank clean up and to reuse to same water.
Ill explain myself, I want to clean the tank, siphon out some water, use some filtration system and have the water flow back in a, lets say a container, and at the end, put back this water into the tank. and ill syphone up all the turtle poop at the bottom with a normal siphon.
Cuz I know changing the water will mess up their ecosystem, so I was wondering its this was a good idea.
and I also know we are supose to change about 1/3 to 1/4 of the water each time.

Unless you are using a filtration system then uses biological media and have a large volume of water, I'd recommend partial or complete water changes.

Your RES are probably already in a tank considered too small for them (1" = 10 gallons). There is a lot of waste and impurities that you can't see in there or separate from the water, thus requiring a partial or complete water change.

Yeah, if you wanna do what your supposed to in taking care of these guys.. A filter is your friend. Otherwise you'd have to change the water I would say AT LEAST twice a week. Maybe more.

I do have a filter, but I was wondering instead of water change is it a good idea to filtrate the water and reuse it wil a external filter soft of?

A partial water change would be a good idea.

The purpose of the water change is to remove nitrates from the water. The nitrogen cycle is ammonia-nitrites-nitrates in that order. Nitrates are the least toxic to your tank inhabitants, but they build up after a while and can become just as toxic as ammonia if they reach high concentrations. The easiest way to remove nitrates from your setup is to do a monthly water change...usually around 10% of the total water volume.

When you are removing old used tankwater you don't know what you are throwing out. Replacing it with clean water is your best bet, then you know what you are replacing it with.. You are much off with clean water from the start. Good luck!

you do know what you are throwing out. Nitrates. The sole concept of a "water change" is to remove nitrates.

The exception is if you don't have a filter... then you're removing ammonia, nitrites AND nitrates.