It is municipal indeed. I realize this isn't exactly a water tight source of information, but my uncle has kept fish since he was around my age and I heard from him that if your water is just left open for 12-24 hours before you treat/use it, it helps with chlorine, something about evaporation. I don't know how true this is, but I've been doing it for a while and it hasn't done me wrong so far, but I'll certainly pick up a bottle of conditioner somewhere.
Chlorine will come out of water by itself but it can take time depending on the level of chlorine in the water. The safe recommended level of chlorine for human consumption is 2ppm, but sometimes water companies get over excited and add more. My water company in Perth regularly has the chlorine levels up around 7+ppm and this is hazardous to anything that ingests it.
Your water company (check their website or phone them) should be able to tell you if you have chlorine or chloramine in your water supply. If you have chlorine in your water supply, and a number of places in the US still use chlorine, you should either use a dechlorinator or put the tap water into a clean bucket and vigorously aerate it for at least 24 hours before use. This makes sure you get the chlorine out of the water. If you don't aerate the water, it can take long for the chlorine to come out. And if the water company has added a heap of chlorine (above the safe recommended limit) it can take longer to get it all out.
Chloramine is a mixture of chlorine and ammonia. It binds together and continues killing stuff for longer than chlorine. Water companies use it in areas where it is hot or the water has to travel long distances. Some companies use it because they think it is safer than chlorine. However, chloramine has chlorine and ammonia in so it's not really safer. Chloramine can remain active for months and does not come out of the water with aeration or letting the water stand for a few days. The only way to get rid of it is to use a dechlorinator that will break the chlorine ammonia bond, and neutralise the chlorine. The ammonia will be left in the water and this can be read as ammonia with a normal ammonia test kit. If the chloramine has not been treated with a dechlorinator, you probably won't see any ammonia readings on a test kit unless the water company has overdosed the ammonia, which actually happens quite a lot.
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Learn something new everyday. I'll be picking up a bottle of water conditioner with haste
When you use a dechlorinator, you should fill a clean bucket with tap water and add the required amount of dechlorinator for the amount of water in the bucket.
eg: the bucket holds 20 litres (5 gallons) of water so you treat it for 20 litres of water.
After you have added the dechlorinator, aerate the mixture for at least 5 (preferably 30) minutes to allow the dechlorinator to come into contact with all of the chlorine/ chloramine molecules in the water.
Aerating the water also allows the dissolved gasses in the water to get back to normal levels. Some of the gasses can be forced out of the water when it is under pressure in the pipes.
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My water naturally hovers around a 7.4, which is pretty alright. My water is incredibly poor in minerals though. Am I still ok using the GH powder, or should I just omit that as well?
I would use the GH powder & dechlorinator but drop everything else. You don't want the GH fluctuating with the water changes.
The reason you want to remove everything else is to make sure there hasn't been a build up of anything in the water. You are adding plant fertiliser and unless it is all used up between treatments, you can quickly get high levels of fertiliser in the water that can affect fish or shrimp. If you use plant fertilisers in the aquarium, you should do a big (75%) water change before adding any more fertiliser. The big water change helps to dilute any left over nutrients and reduces the chance of you overdosing the tank.
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If the water changes and lack of additives fixes the problem and you don't lose any more shrimp, then start adding a bit of plant fertiliser and see how they go.