No, too much rain water can affect the pH level of a fish pond which in turn could affect the health of your fish.
Although you might think rain water is a wonderful source of free unchlorinated soft water for your pond you have to take other things in to consideration.
The area you live can have a big effect on rain water, a big industrial location could be putting out lots of pollutants in to the air, when it rains the pollutants are brought down with it, this results in the rain water turning acidic, high volumes of acidic water in your pond could cause a pH crash, and it’s not just industry you need to worry about, big cities have urban pollutants as well as emissions from cars, even farms can have an effect on rain water if they use fertilisers and pesticides.
The size of pond can really be affected by rain water, the smaller the pond the more it will be affected, this is due to how the pond water is able dissolve the amount of rain that has fallen, the bigger the pond the quicker and easier the rain water is dissolved.
Collecting rain water from your roof in to barrels might sound the perfect solution for your pond, but you have to take into consideration that not only will the rain be acidic but the roof itself could have a build up of harmful contaminants that will be washed off the roof and in to the barrels, if you are set on using rain water in this way you will need to filter it first.
Unless you have a covered pond it’s impossible to avoid rain water entering your fish pond so you need to do regular parameter checks of the water to determine the pH value, if the values are outside the ideal pH reading you need to use a treatment to get the values back on track.
Always check your ponds pH level after a deluge of rain or after a prolonged period of rain, this way you will be able to keep your pH values in range with treatments before the health of your fish are affected.