I guess as long as the rain isn't too acidic it would be OK. Since it was a gradual change in pH (assuming) no harm. Then again I have dropped pH in a tank rapidly in a CO2 injection incident and there where no visible signs of stress or any deaths afterward. This must happen frequently in the tropics where most fish come from. I have seen local ponds and lakes overflow and no floaters afterward. If it ain't broke - don't fix it.

Hi
Thanks for the reply. I've always been a believer in water changes but was always done by hand so the percentage
was always controled. What percentage and how frequently would you think is too much..?? On a heavy rain I figure it could be 700 times the volume of the tank.
A simple valve of course would stop it but wouldn't be automatic.. Another thing I need to do is bypass for a few minutes to remove whatever is on the roof before changing but can't come up with an idea on this thanks gary

What percentage and how frequently would you think is too much..??

Ideally, I like 30-40% once a week. Do I always do that? Thats another question.

A simple valve of course would stop it but wouldn't be automatic.. Another thing I need to do is bypass for a few minutes to remove whatever is on the roof before changing but can't come up with an idea on this thanks gary

I am not sure what the setup looks like. Would a flush valve with a float or ballcock ork? Like in the toilet tank???

Hi
The rainwater comes from the roof into gutters a two inch pipe carries the water from the gutter under the gravel where it is released by a spraybar . The water then overflows in to a standpipe where it is carried to the lily pool .This overflows into a bog garden at ground level. Used a two inch pipe as it fit into the gutter fittings. An inch and a quarter for the standpipe but will not handle a 1 inch rain. which causes overflow in the tank. Was more than adequet before adding the downspout.
Simple gravity is the power method. Would prefer the water to go into the lily pool first then into the tank then to the bog garden. But the tank is the highest.
I'm rather surpised at the volume as I'm only collecting water from one side of the hip and only a third of the roof area. Wasn't sure how to estimate how much water would actually flow into the pipe. WAAAY more than I thought.lol
Wanted to keep it as simple as possible. In order to have any kind of control ,obviously need some type of reservoir though i think I'll try a valve on the downspout first. if I can figure out what position to set the valve.lol Worked fantastic for a half inch rain in 1 hour duration .
They make bypass valves for cistern water systems which can be set at certain volumes before water is allowed to collect. Allows washing the roof so you get only the cleanest water. Guess that solves that problem but was sure hoping not to need a reservoir.
Thanks for the help. gary

Maybe re-plumb it? Off the rain gutter downspout use a 2" TEE. Have the TEE face to the side, rather then up ( I hope I explained that right). The part of the TEE that isverticaly in-line with the down spout, reduce to 1" or 1 1/4". The horizontal part of the TEE can remain 2" and just drain out to the ground or the garden or where ever you like. This will be your way to control how much water goes to the tank first. The overflow from the tank to the other ponds etc. should be a larger diameter then the supply pipe from the gutter. So, if the supply is 1" then the overflow from the tank should be greater to handle the flow. You should be able to check on a web search the maximum flow rates of different diameter pipe. I think that the diameter of the "drain" pipe should be about 3-4 times the diameter of the supply pipe. Like a tub supply is usually 1/2" and the drain is 1 1/2" to 2". Sorry if I'm way off but kind of hard to tell without visuals. Sorry for my usual rambling =0)

First thing first: this would is not safe. Even in the most remote area rain can be acidic, not worth the risk. Also changing more than 25% your water every two weeks is a bad thing. The bactria (the guy) that eats waste etc. wuold be wiped away. I would suggest not using this system, because of the risks. The water could have touched anything!

Celly if that where true all fish in outdoor ponds would die after a big rain storm. Alot of tropical streams and bogs are in areas where monsoons occur. These areas are also where some of our fish came from and live, Betas included. The bacteria lives in filter sponges/media and in the gravel. Actually the bacterias that you are talking about need water flow to survive. The bacteria feeds on the ammonia and nitrites in the water that flows through it. I do as much as 60% water changes weekly (sometimes 75-80%) depending on the tank and never had a bio-filter crash.