# 20 Gallon TankHow do I know how much a 50% water change is?

rkeehn547
Hello, I’m wanting to do a 50% water change per week for my planted aquarium but I don’t know how much water to take out. I have a 20 gallon tank and when I took out 10 gallons of water last week my nitrates went from 30ppm to 10ppm which means I really took out 65-70% of the water(I’m assuming because of the gravel and decor in the tank taking up space). Is there any way I can figure out exactly how much water to take out to equal 50% of the water volume? Thanks

StarGirl
Hello, I’m wanting to do a 50% water change per week for my planted aquarium but I don’t know how much water to take out. I have a 20 gallon tank and when I took out 10 gallons of water last week my nitrates went from 30ppm to 10ppm which means I really took out 65-70% of the water(I’m assuming because of the gravel and decor in the tank taking up space). Is there any way I can figure out exactly how much water to take out to equal 50% of the water volume? Thanks
I wouldnt worry, I would just do the 2- 5g buckets myself.

TClare
It is not that critical, just take out enough so it looks as though the tank is just over half full (to account for substrate etc.) It doesn't matter if it is a bit more or less.

PAcanis
I saw someone say to pay attention to how much water it takes to fill your aquarium after scaping it. Then you truly know how much volume liquid is in it.
But who does that? lol

Yeah, best guesstimate here. I pick a spot when siphoning and hold the hose there until I hit it.

Dunk2
I agree with the other posts above. There’s really no need to be this precise, but. . .

If you want to get close, measure the distance between the water line and substrate. A 50% change or thereabouts will be in the middle. Make a small mark on your tank at that point.

Hello, I’m wanting to do a 50% water change per week for my planted aquarium but I don’t know how much water to take out. I have a 20 gallon tank and when I took out 10 gallons of water last week my nitrates went from 30ppm to 10ppm which means I really took out 65-70% of the water(I’m assuming because of the gravel and decor in the tank taking up space). Is there any way I can figure out exactly how much water to take out to equal 50% of the water volume? Thanks
What most people don't get is, a 20 gal tank (any size tank really) does NOT hold that amount of water! No one ever fills the tanks to the tip top! So, the best you can do (or what I do) is, I take a ruler and go by inches. I mark the 50% and 75% areas, from where the top of the ruler is from the bottom. (Half way up to the full line and 3/4 of the way up to the full line where I have the full water line.) Then, I remove the water down to that line, depending on if I am doing a 50% or a 25% water change!

Makes things easier for me! Hope it's helpful to you!

RayClem
A 20 gallon tank holds "approximately" 20 gallons when a bare tank is filled all the way to the overflow point. Once you add gravel, rocks, decorations, etc. and you fill it to 1/2" from the top, it may hold only 17-18 gallons.

There is no magic in doing 50% water changes. There is certainly nothing wrong with changing 50% or even slightly more. However, doing something like a 75% water change might alter the water quality to the point it becomes stressful on the fish. Even a 50% water change would be stressful if you have not done one in a couple of months. If a tank has been neglected for weeks, it would be better to do a 10% water change every couple of days rather than a single large water change. The main thing is to change a portion of the water on a consistent basis so that toxins do not build up in the water. There are many ways of accomplishing that goal.

StarGirl
Filling the tank all the way to almost the top also gives you your max water capacity and makes for no water lines up top of your tank. I think mine are a 1/4 inch from the top when I refill.

mattgirl
I agree with the other posts above. There’s really no need to be this precise, but. . .

If you want to get close, measure the distance between the water line and substrate. A 50% change or thereabouts will be in the middle. Make a small mark on your tank at that point.
This is what I do. I have a mark at 50%. I remove 24 gallons out of my 55 gallon tank when I do a 50% water change. This tells me my 55 gallon tank is actually only 48 gallons of water.

Hello, I’m wanting to do a 50% water change per week for my planted aquarium but I don’t know how much water to take out.
Close to 50% is good enough. A little more or a little less isn't that critical

KrissyBunnie
To be honest, I know it sounds silly but I've actually taken out a tape measure. I start at the bottom of the tank and stop at the surface. I believe mine is 27 inches tall, so I just stop after it gets to 14.

PAcanis
The real problem I have with splitting the difference between your substrate and your waterline and calling it 50% is, what if your tank's substrate raises towards the back of the tank a lot?
And I've noticed that water poured into a sand substrate seems to saturate the substrate more than poured into an Eco-Complete substrate. At least it takes me more water to fill the tank with a sand bottom.

I think it only makes a difference for medication... not.
Because some of the meds say it's OK to increase dosage by 50% or change once a day to twice a day.
And if aquarium meds are anything like the meds for our canines, they are tested at 10x the recommended dosage.

So bottom line is, there's some leeway.

mattgirl
To be honest, I know it sounds silly but I've actually taken out a tape measure. I start at the bottom of the tank and stop at the surface. I believe mine is 27 inches tall, so I just stop after it gets to 14.
Not silly at all but it would be easier just to put a dot with a permanent marker at the 14inch level. this way you only have to pull out the tape measure one time.

Fishnturtleguy933
the advice on this thread is more than sufficient and you couldn't go wrong with anything thats been said. But, just to add too it. I usually just eyeball it as I'm emptying it. I use a sump to remove water from my tanks and it's easy enough to stick it to the side of the tank at roughly halfway. This wouldn't be ideal for smaller tanks though. I also never gravel vac, rooted plants take in all the nutrients. I've found for me, so long as it's no more than what appears to be half the tank works, roughly 50-60% but i don't go more than that unless there is an emergency or i need to move a tank to the other side of the room to make space for more tanks....lol

Arvil
As they say, it ain’t rocket science. Mine is also a 20g. My ammonia, nitrites, nitrates are consistently zero, just getting rid of other gunk. A 3 gallon bucket 3 times is close ‘nuff.

KrissyBunnie
Not silly at all but it would be easier just to put a dot with a permanent marker at the 14inch level. this way you only have to pull out the tape measure one time.
Definitely going to be doing that... Thanks!

Azedenkae
Hello, I’m wanting to do a 50% water change per week for my planted aquarium but I don’t know how much water to take out. I have a 20 gallon tank and when I took out 10 gallons of water last week my nitrates went from 30ppm to 10ppm which means I really took out 65-70% of the water(I’m assuming because of the gravel and decor in the tank taking up space). Is there any way I can figure out exactly how much water to take out to equal 50% of the water volume? Thanks
Assuming you did not have nitrite, the answer to your question is 50% is approximately 6.66 gallons.

Replies
6
Views
300
Flyfisha
Replies
15
Views
186
SirFinn
Replies
7
Views
501
LinasPlantLife
Replies
16
Views
607
SuperSword48
Replies
4
Views
60
donnabright2002