Water Changes Question

[FishySarah]

I was just wondering what people think. Which is better-

20% water change once a week

or

10% water change twice a week?

Personally, I'd rather to the smaller change more frequently, but would it be ok for the fish?

 

[minxfishy]

What size tank are we talking about here?

 

[FishySarah]

What size tank are we talking about here?

See sig file

 

[Davo86]

What size tank are we talking about here?

See sig file

2 1/2 gallon - - 1 x 50% per week,
1 Betta

1 gallon - - 2 x 50% per week,
1 betta

5 gallon - 1 x 25% per week
1 betta

29 gallon 1x 25%
3 platys
5 guppies
1 adult and 10 juvie mollies

10 gallon - 2x10% or 1x20%
3 gold barbs
3 black longfin tetras

10 gallon (for now) - 2 x 50% per week, (take it you know this isn't ideal)
4 comets

5 gallon - 1x25%
tons of guppy and molly fry
2 livebearer fry of unknown origin given to me by pet store.

Two cycling 10 gallon tanks

 

[FishySarah]

Wow, that's really helpful, thanks. If no one disagrees, I will print that out and save it.

As for the comets, they were originally feeder fish for my tai devil crab, which then upped and died on me, leaving me stuck with them. So they weren't really a choice on my part- but I'm kind of attached, to be honest. They are still little.

I have a friend who has a pond and I will probably give them to her when they outgrow the tank I have them in. If not I will either have to upgrade or find another home for them eventually.

I hope they will be ok through the winter in the tank though.

 

[OldMan47]

The twice a week 10% is not the same as once a week 20%. Multiple changes can be thought of asa product rather than a sum. 90% of the water left behind, then 90% of that left behind is 81% of the original water still in the tank, not 80%. For extremely small changes like you propose, the difference is not too great but if you start working with larger percentages, the difference becomes more obvious. A 25% followed by a 25% gives 75% of 75% remaining or about 56%. That means you have done something similar to a 44% change, not a 50%.
Each of your tanks should get a nitrate test shortly before the water change. If levels go up from one week to the next, you are not changing enough water. If readings go down from one week to the next, you could afford to cut back a bit. Each tank needs to be judged based on its individual biological load as that will determine the real water change needs. The easiest thing to adjust would be the amount of water changed rather than the how often so keep that in mind when you are making the adjustments. Adding fish will mean more water needs to be changed if you are doing just enough, so I would tend to make somewhat larger changes than I really need to. That way new fish will not require you to adjust further.

 

[Jazzzz]

Wow, that's really helpful, thanks. If no one disagrees, I will print that out and save it.

As for the comets, they were originally feeder fish for my tai devil crab, which then upped and died on me, leaving me stuck with them. So they weren't really a choice on my part- but I'm kind of attached, to be honest. They are still little.

I have a friend who has a pond and I will probably give them to her when they outgrow the tank I have them in. If not I will either have to upgrade or find another home for them eventually.

I hope they will be ok through the winter in the tank though.

i agree with this

 

[backtotropical]

2 1/2 gallon - - 1 x 50% per week,
1 Betta

1 gallon - - 2 x 50% per week,
1 betta

5 gallon - 1 x 25% per week
1 betta

29 gallon 1x 25%
3 platys
5 guppies
1 adult and 10 juvie mollies

10 gallon - 2x10% or 1x20%
3 gold barbs
3 black longfin tetras

10 gallon (for now) - 2 x 50% per week, (take it you know this isn't ideal)
4 comets

5 gallon - 1x25%
tons of guppy and molly fry
2 livebearer fry of unknown origin given to me by pet store.

Two cycling 10 gallon tanks

I don't agree with this. Every tank is different and there are so many variables, it is impossible to give an accurate estimation. Whilst the regime quoted above may work for one person, it may be inadequate for someone else.

A good basis to base your water changes on is to monitor nitrate levels both in the tank and in the tapwater. Perform sufficient water changes to keep the nitrate level in the tank as close as possible to the nitrate level in the tap water. If the nitrate level in the tank is substantially higher, that would indicate that nitrate is being allowed to accumulate and therefore larger or more frequent water changes are required.

You may find that nitrate levels don't rise much at all. In that case, don't neglect water changes as they have many more uses than removing nitrate, but do a minimum arbitrary amount of say 25% per week.

Again, this isn't gospel, but is a better method than picking arbitrary amounts out of the air based on the size of the tank and the stocking levels.

The crux of the matter is that there is no right answer, however basing the water changes on nitrate level has some credibility.

With regard to your original question, Oldman47 makes a crucial point. Remember that 2 x 50% changes isn't equal to 1 x 100% change, but instead only 1 x 75% change. So long as you are changing some water on a regular basis and nitrate levels are staying in line with the tap water, I'd suggest that how you do the water changes isn't really important.

Hope this helps you.

BTT