Yes, it's going to cause a problem. When they do work on the water pipes, they add extra chlorine or chloramine to the water to make sure nothing is alive in it. It can take several days for this heavily chlorinated water to get used up by other properties and back to normal levels of chlorine/ chloramine. However, if they are going to be doing it over a 4 month period, you could poison your fish.
You need to find out if your water company adds chlorine or chloramine to the drinking water. If they add chlorine, you can fill up some containers with water, add a double dose of dechlorinater and then aerate it vigorously for a few days before using it. You can do a similar thing with chloramine but it doesn't come out with aeration so you should also invest in a chlorine test kit (available online or at pet shops). Test the chlorine level before adding dechlorinater and again an hour or so after you have dechlorinated it. If there is still chlorine present then add more dechlorinater, aerate it and wait another hour before testing it again. I'm not sure if a chlorine test kit will read chloramine so that will need some research or experimenting.
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Another option is from Whiskyfish, start storing water now and use it when they start working on the pipes.
If you have friends or family that live somewhere that isn't getting the water pipes replaced, you could visit them once a week and get some tap water from there to use on your tanks. It's time consuming and strenuous carrying 10 or 20 litre bottles to the car but it might be an option.
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A reverse osmosis (r/o) unit might help but the carbon filter could end up becoming full quite quickly due to the high levels of chlorine/ chloramine in the water.
A reverse osmosis unit consists of a number of filters that physically remove dirt and minerals from the water, and then they filter the water through carbon and possibly other substances to remove chemicals and heavy metals. The water goes in one end and there are two outlets. One outlet has the pure water in and you collect that for the fish tank. The other outlet contains the unwanted minerals and this is considered waste water that usually goes down the drain or onto the garden.
Reverse osmosis units are slow to produce any water and big units can put out 10-20 litres an hour but most smaller units do less. They also use a lot of water. When looking at r/o units you look for the waste water vs pure water ratio. The good units will have a 1:1 ratio whereby they produce 1 litre of pure water and 1 litre of waste water. Cheaper less efficient units might have a 2:1, 3:1 or 4:1 ratio whereby they get 1 litre of pure water to 2, 3, 4 or even more litres of waste water.
If you use r/o water, you might need to buffer it up for the fish. R/o water is free of minerals and if you keep livebearers (guppies, mollies, platies, swordtails), rainbowfish, goldfish or African Rift Lake cichlids, you will need to add some mineral salts to the r/o water to increase the hardness.
If you keep tetras, angelfish, gouramis, barbs, rasboras and Corydoras, you won't need to add anything to raise the hardness.