Big Water Changes

[DazedNCoNfUsEd]

This is just something I was wondering about. It's recommended to 30% water changes weekly, but would it do any harm to do for instance a 60-70% water change? I thought about this while researching fishless cycling. At the end of the cycle right before you add fish you have to do a 90% percent water change, and that obviously doesn't affect the cycle at all, so why would it affect it after the tank is up and running with fish?

 

[Fat Boy]

The biggest problem with doing a huge water change is the stress and shock it can put on the fish, big, fast changes in water conditions for ANY fish isn't good.

Unless you have MASSIVE problems ( i.e high levels of ammonia or NitrItes ) I would'nt do more than a 30% change in a day.

 

[andywg]

It depends on the difference between your tank and the tap water. Ideally you don't want to do more than 50% water changes, but if the pH is pretty much the same, as are the other levels (including temp) then I don't see anything wrong with it.

For general maintenance though, 30% is a good guide. I try and keep to 30% because then I don't have to worry about the temperature of the water going into the tank and can link the hose straight to the mains.

 

[DazedNCoNfUsEd]

Okay thats what I figured. I just remember reading somewhere that you remove too much bateria by doing a big water change, but since the bacteria is hardly in the water I didn't think it would matter. Just wanted to clear things up though. Thanks guys.

 

[Bignose]

The 2nd and 3rd posters are right in that if you change the water conditions it can be bad, but if you set it up where the temperature, pH, and hardness of the new water is the same as the tank water, you can do almost as big a change as you want. Consider nature: continuous 100% water changes. You are also right that the overwhelming majority of bacteria are sessile (which is the opposite of mobile). The bacteria are in the filter and on every surface in the tank. So, again, replacing water does nothing to affect them. I also agree that normally a large change should not be necessary, but it is good to know that it is ok in case of emergency (like the fish reacting poorly to medicine)

As far as I know, there are really only 2 occurances when a large water change should not be done 1) When the tank pH is very low and there is lots of ammonia. If the pH gets raised too fast, it will change the ammonia-ammonium balance toward the more harmful ammonia and 2) if you have an undergravel filter and you root around in the gravel a lot when you change the water. This may disturb the bacteria bed. But, even with an UG filter, you can just suck water out without digging around in the gravel.

 

[freddyk]

Anybody who does their regular weekly water changes can safely do large water changes on their tanks, as long as the temeprature introduced is right. The PH shouldn't be far off on a well maintained tank, and if it is there's other problems. The biggest fear for large water changes is the neglected tank - nitrates and other toxins build up, the ph drops, and the fish adapt to these gradual changes. In these cases the sudden change to good water can and likely will shock the fish.

 

[Conqueror]

As long as you don't have awful chemicals in the water, large changes aren't necessary. Three 30% changes on consecutive days are just as effective, and stress the fish less. The main "stress" I see in water changes isn't the water conditions, it's the water VOLUME. Doing a 70% water change means you remove 70% of the water - thereby cramming all your fish into a space that's only 30% as big as they're used to. That's gotta be stressful!

CQ

 

[Bignose]

As long as you don't have awful chemicals in the water, large changes aren't necessary. Three 30% changes on consecutive days are just as effective, and stress the fish less. The main "stress" I see in water changes isn't the water conditions, it's the water VOLUME. Doing a 70% water change means you remove 70% of the water - thereby cramming all your fish into a space that's only 30% as big as they're used to. That's gotta be stressful!

CQ

Actually, that's not precisely right. 1 70% change reduces the concentration to 30% of its original value:
1*(1.0-0.7)= 1*0.3 = 0.3 or 30%
3 30% changes would be 1*((1-0.3)^3) = 1*(0.7^3) = 0.343 or 34.3 % of the original concentration. Not to mention if there is any production over the 3 days. If better means reducing the concetration, than 1 70% change is better, however, all the conditions I and several other have already written need to be heeded: the new water has to have the same temp, pH, and hardness of the tank water.

 

[DazedNCoNfUsEd]

Yeah that is one of the main problems, cramming all of your fish into a small space.

But like Bignose said it is good knowledge to have in case in some emergency or something where you have to do one, just one less thing to worry about.

Okay Bignose I agree with your first post but have no clue what youre talking about in that last one.

 

[Bignose]

Let me try to explain the math a little better then:

A primary goal of water changes is the lower the concentration of pollutatants in the water, e.g. nitrate.

Let's say that the current nitrate reading is 100 ppm. ppm is parts per million which is another way of saying 100 mL of nitrate per 1000 L of water. or 100 microliters of nitrate per 1 L.

Just to make the math simpler, let's say the tank is 1 L, so if the reading is 100 ppm, there is exactly 100 mcL (mcL = microliter) of nitrates in there. Now, lets do a water change of x% This reduces the amount of nitrate by that x% (just like a sale)

And, so long as you put perfectly clear water (no nitrates) you there won't be any extra nitrates in there.

So, let's look at the 70% water change (x=70%)

100 mcL to start, 70% goes out. 70% of 100mcL is 70mcL so
100-70 = 30 mcL. or 30% of the original concentration.

Now, let's look at the 3 30% changes

100 mcL to start. 30% goes out. 30% of 100mcL is 30mcL so100-30 = 70 mcL. This is the concentration after 1 water change.
Now, let's do another 30% change. 30% of 70mcl is 21mcL. so 70-21 = 49 mcL. This is the concentration after 2 water changes.
One more time 30% of 49mcL is 14.7mcL. So 49-14.7 = 34.3 mcL which is the answer above. Also it is 34.3% of the original concentration, less than one large 70% change.

This is where my comment about production comes in. If each day there is production of nitrates, this math requires a little modification; what ever the generation rate is needs to be added after the dilution step but the idea is the same.

The math in the post above is just the quick way of doing that caclulation without stopping at each point (that is what the powers do in this case -- repeated water changes). A little math shows that 3 33.1% water changes is equal to 1 70% change.

If you want to look at the equivlance of two schemes, here is the math:

1 x% water change versus z y% changes (that is perform a y% change z times)

(1-x/100) = (1-y/100)^z

The way I found the 33.1% above is by solving for y after setting x=70 and z=3


You can even generalize this more, w x% changes versus z y% changes:

(1-x/100)^w = (1-y/100)^z

Using this we can find the 3 30% changes are equal to

2 changes of 41.4%
4 changes of 23.5%
5 changes of 19.3%
6 changes of 16.3%
8 changes of 12.5%
10 changes of 10.1% and etc.