When changing the water in my fish tank, why is it necessary to leave about 25% of the old water behind?

Answer 1

It really depends on how crowded your tank is to determine how much and how often you should change the water in your tank. If you have a well balanced tank, 1" of fish to 1 gallon dispersed, then you really should only need about a 30% water change once a month.

After that, the rest of the people on here are accurate. It is to keep a balanced water level, keep fish from a dramatic change in water to prevent shock, and keep bacteria that keeps your tank clean.

One more thing, you should make sure you are putting treated water or distilled water back into your tank.

Answer 2

When changing water you only do partial changes, but when cleaning the tank you can change all the water BUT!!!!
Remove the fish and put them in a bucket with the same water that is in the tank, so that the fish don't go into shock. Remember a fish does not like fresh water, the water in the tank is already stale and that's why the fish thrive in it. You must clean the tank properly boil the stones or what i used to do is rinse the stones under running water until the water runs clear, that way you are not rinsing all the bacteria away either, but rinse until most of the gunk is out, and please whatever you do DO NOT use metals when cleaning anything that belongs in the aquarium as that will also kill the fish. Your best bet is get a BIG bucket or bath plastic and run water in there, and let it stand overnight, making sure that the water becomes stale. Use this water first, then throw the water in that the fish is in and then top it up with tap water, run all filters and make sure that there is enough oxygen in the water before putting the fish back, that way you won't lose any of your fish, and will assure a healthy tank.

Answer 3

The main reason to leave some behind is preserve the bacteria which convert ammonia into less toxic chemicals. Generally it's a bad idea to change more than 1/2 of your water at once. (Goldfish and bettas in bowls are pretty much the sole exception to this.) Changes in PH, temp, and hardness can be cause a great deal of stress to your fish. You should really just use a cheap plastic gravel vacuum the gravel, and remove 10-20% of the water every week. Then once a month remove 40-50% of the water.

Answer 4

i never leave 25% in my tank when I'm cleaning it out and i use normal tap water with out any other chemicals when i fill it back up, my fish are still alive and have been for over a year,every time i done it like they experts say leaving 25% in the tank and using all these chemicals to get the balance right in the water my fish dies or got a disease so i just do it like i said.depends what kind of fish you have!

Answer 5

because the fish need the nuitrents that are left in the remaining water
you can however take the fish out and put them in the old water in a container and change all water in the tank as long as you leave it stand for about an hour and make sure you put nessarily treatments in before adding your fish
I have a tropical 4 foot tank and have done this on numberous occassions and I've had no fish die when I have done this and I have had the fish tank for 5 years now
hope this helps you

Answer 6

Every fish is different, and so is every tank. On occasion I have removed all but 5% of the water, to clear it properly, but now I am in a routine once a fortnight I remove 2 pints of water, and replace with 2 fresh pints of water (I have a goldfish) using aqua safe. The fish is lively, and active. It also depends on food, get a sinking food as pockets of air slowly bloat the fish and upset the balance of his swimming. If you use sinking food, then you'll have to clean out the stones/gravel at the bottom of the tank which results in removal of a majority of water. I personally mix bloodworm and powdered food, bloodworm once every three days, and a small pinch of powdered food every morning.

Answer 7

leave 25% of old water BEHIND???

This means you are changing 75% of the water? You are taking FAR too much water from your tank in one time.

You should change about 20% of the water each week or so. The old water neds to saty as it is mature and contains benficial bacteria to break down waste products in your tank. If you add too much new water, you will destroy all the beneficial bacteria and your tank will become toxic more quickly

Answer 8

I never heard that before. But if you get a new fish you have to keep some of the old water to gradually change the fish to your new water chemicals.

Answer 9

It doesnt matter as long as when you empty the tank, your fish are in another container with water. If you leave 25% of the water in the tank, make sure that you dig deep in the rocks/sand to uncover that nasty fish poop!

Answer 10

for the bacteria the fish they can't be in totally clean water,also setting up the tank 24 hours prior to putting the fish in helps the chlorine and any other chemicals in the water evaporate.