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Okay, so we decided to move this elsewhere!

Question up for debate: when is there a need for very large (70% or more) water changes in a cycled tank? Are they ever necessary? Are they ever completely out of the question? Can a newbie handle them? An experienced aquarist? Can they help? Can they hurt? Can you maintain a tank using just these, and never the "normal" size water changes? An overstocked tank? A tank with water quality issues?

-Molly
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I explained this in another post somewhere...

In the "old" days before the cycling process was in vogue, large water changes were the norm.

But, these changes were done with the same temp water and sometimes, but not often, the pH was neglected.

Results varied with the pH. Sometimes it matched, sometimes it didn't. When the match was too far off the fish reacted violently and died.

A bacterially balanced tank, cycled, never took place because the tank water never got a chance to cycle. The large and sometimes frequent changes prevented that from happening.

Keep in mind that filtration was not what it is today. It was weak and sometimes non-existant thus one reason for large water changes. The tank water would get extremely smelly and all shades of colors after awhile.

If you want, Molly, you can practice this method of fishkeeping. But let me warn you it is extremely difficult, not to mention hard work, to make large water changes of the same temp and pH, but it can be done.

I'd much rather get the good bacteria to do the work for me by just providing small water changes now and then along with the modern filtration systems we have today.

When doing a large 75% water change in a cycled tank you will create a new cycle unless there's enough good bacteria in the gravel, on the glass, on the filter media and on anything else that's in the tank to help recover the missing free-floating bacteria that was in the water you just removed.

The Pakistani I mentioned in another post you read, falsely believed the poster in Australia had chemed his tank to death and told the Australian to do a 75% water change to remedy the chem problem and cure his dying fish.

I of course reacted adversly and told the Australian to do some small water changes and not to listen to the Pakistani. The Australian had a full bio load in his tank and I was certain the tank would mini cycle and overload with ammonia causing all sorts of problems.

The Pakistani practiced frequent large water changes and of course had no knowledge of the nitrogen cycle.
----
A bacteria starter should be dosed just in case the remaining bacteria colonies are not properly developed after performing a large water change.

--garyroland.



[span class="edited"][Edited by garyroland 2004-08-29 13:34][/span]

I don't personally use this method, so please leave my maintenance schedule out of it; we're debating the merits of very large water changes in assorted situations, and when or if they're warranted. Here's a question for you, Gary - in what context, if ever, would you recommend a 75% water change?

-Molly
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I know you don't use that method, Molly...

Large water changes take place when a tank is poisoned or loaded with a high concentration of ammonia/nitrites that cannot be controlled by small water changes.

The fish would have passed away long before the large water change under those conditions.

--garyroland.

Everyone knows large water changes are standard with some discus keepers / breeders, I've heard of some who change 100% of the water daily.
Whether this is due to poor filtration, both biologically and mechanically, on grow out tanks or replacing minerals etc is debateable imo, but I can't really argue with results.
I'm pretty sure some species of fish do demand the higher changes though, rays are the one that spings to mind most, but thats more due to the fact that they can't handle even low levels of nitrates.

On a tank where the species didn't demand it, there are too many things that can go wrong, as Gary perviously stated, with the PH, temps etc.
Personally, the only time I would do a change of that size, in one go, would be if toxins somehow had entered the tank.

Kenny

I do 50-75% water changes in all my tanks either weekly or bi-weekly. All of my fish are active, healthy and show awesome color. Keep in mind however, that most of my tanks are over-filtered (if there is such a thing) and I'm sure the extra filtration (and hence the extra bio-media) are able to handle the bioload with these large changes.

I just wanted to point out though that 50-70% weekly water changes are recommended for puffers by all the leading puffer experts. Heavy filtration is also recommended. If fish as sensitive as puffers can handle these large water changes, then I don't see why other, hardier fish would be at risk. I could understand if the tank was under-filtered, but if filtration is adequate to good then the tank and fish, imo, should be fine.

Just my $0.02

Jason

you mentioned large scale water changes but didn't talk about the time....it can be 1 hr..2 hrs....10 hrs....as Kenny mentioned, 100% water chage daily for Discus breeder is common, but I think they do it in a "drip" and "drop" method...so water goes out slowly and go in slowly in many hours time and reaches 100% water changed....in this way the water can be kept in high quality (too slow for removing poison though) with acceptable environment change rate

I didn't bother reading the rest of the messages so this might have already been covered.

If you match the temperature and pH exactly (temp can be off a degree or so) then there wont be a problem. I would only do a WC that big if I had major water quality problems.