How often can I do a partial water change for 29 gallon tank in an emergency?

Nitrites are dangerous becuase it adheres to the hemoglobin in the blood cells blocking the transfer of oxygen into the cells to make energy accessible to the fish. Mollies are somewhat sensative to changes in water chemistry, I think that's a good suspect to what killed them.

That's a good sized change. I think you're taking the right steps to solve this problem. Keep one thing in mind though. Nitrites are the second part of the nitrogen cycle chain, and you do want to have some present for the proper bacteria to grow, so in the future, it will not be present in your tank, it will just immediately get oxidized. At 2.0 ppm though, thats still high enough to do some damage. In here, I've seen many advocate different advice on water volume changes, and though some might not do what I do and it'll work for them, just like there are 7 foot people on the earth, there are always exceptions so I won't say in this case, someone doing different is wrong. But I really wouldn't advise you to change out more then 50% of the total volume in your tank at any one time.

Several problems have the potential to occur changing out volumes greater then half. One being a massive ph shift, becuase the ph in your established tank, in more cases then not, is not going to match. Shifts over half a point can do just as much damage as the nitrites would do. The temperature is another issue as well. That much influx of fresh new water, is definetly going to influence the overall temperture of the tank. It's almost impossible for you to take a tap source or any other source and get it to match the tank temp exactly, and in higher volume changes, you effect more, the overall temp of the water. To much of a shift could cause your fish to go into shock.

At this point, the fish have been exposed to the nitrites so far, and doing your 30% change gave them some relief. You can help keep the nitrites lower by reducing food amounts and feedings too. Either way, more food means more waste, whether eaten or not. You also don't want to totally 0 out the nitrites either, because your tank does not have the proper bacteria levels to cope with it. Brining it down under a 1.0 reading should keep it safe, and provide enough nitrite present to assist the bacteria to grow out enough to oxidize any incomming nitrite immedatly, and sovle the real problem. Just be patient, give it some time, and keep doing your partial change outs :)

JV

In a small tank the water parameters can fluctuate more quickly than a larger tank. The water will get polluted quicker therefore you will need to do more frequent water changes. For good tank maintenance, you should change 20% of the water weekly in a larger tank (what I do for my 30g and 20g). With a 5 gallon tank, I'd do a 20% change twice a week. You could even change a little more if you want. The beneficial bacteria lives in the gravel and on the filter media so unless you disrupt that you will not have a problem. Remember to dechlorinate any new water you add. You can get a test kit for ammonia, nitrite and nitrate. Your ammonia and nitrite should both be 0, and the nitrate should be below 40ppm. If any of those numbers are off, you need to do more frequent or larger water changes. Smaller, more frequent changes are better for fish than large, infrequent changes.

I want you to take my advice please. Changing water NEVER hurts the fish, if you keep the temperature the same. 30% is not enough to do much good. You can change 50-75% daily if you need to. Nitrites can kill your fish and are still too high. You are going to have to change a lot of water at first, half of the tank water if ammonia or nitrites get to 1ppm. Don't touch the filter until you see green algae starting to grow on the glass, just replace the carbon every three weeks. So change all of the water but leave the filter alone. Cycling the tank occurs in the filter NOT in the water. The water must be clean or the fish won't make it.

I would keep doing it about 1-2 times a day for a week or two to make sure you get your nitrite level straight.

The only reason you would have nitrites in there, would be if your tank is still cycling

Are you sure it's nitrites and not nitrates?

Have you checked your ammonia levels?

EB

generally i wouldnt do it more than once aday, while you want to get them out of the bad water fast, thatcould actually kill the fish, as the shear shock of the difference of the water.

So once a day, you can do more to, 30% isnt reallyanythinhg,

i always do a 70%-50% and have no problems. but thats every week.

Ive done it everyday when I rotate fish.

Still trying to get everyone perfectly matched in my tanks.