You want a product that states that it removes ammonia. Chloramine is a combination of chlorine & ammonia. Sodium thiosulfate, which is in all dechlorinators, and is usually the only component in the cheapest dechlorinators, will take out chlorine, and split the chlorine/ammonia bond in chloramine, leaving you with ammonia. You already know ammonia is not good, so do the manufacturers of your better water conditioners. To deal with this they use hydromethane sulfinate, which converts ammonia to ammonium.
Ammonium is harmless to fish at the levels found in an aquarium, but used the same as ammonia by your nitrifying bacteria. Your bacteria have two different types, one that converts ammonia into nitrite, and another that converts nitrite into nitrate. If your water supplier jacks the chloramine for whatever reason, and your water conditioner doesn't deal with ammonia, you will get an ammonia spike. Often, when there is less chloramine you can get by with a cheaper sodium thiosulfate dechlorinator, letting your nitrifying bacteria deal with a small short term ammo spike. If they hose the water with chloramine the increase in ammonia can cause problems.
Along with this increase in ammonia, which can be converted to ammonium, may come a nitrite spike, as your nitrifying bacteria, which is converting the excess ammonia/ammonium is temporarily overwhelmed converting the nitrite to nitrate. This is usually a short term situation in a mature tank, and will clear itself up within 24 hours.
These are the reasons that it is a good idea to learn as much as you can about your water supplier. Generally they will dose higher late in the evening during the week, in anticipation of increased water usage in the morning as folks get ready for work. Knowing this, the best nights to change water are Fridays & Saturdays, the worst is Sunday night. Any weekday around noon or so is good for water changes. This does not take into account any seasonal or weather affects on your water supply.
The main reason not to trust your water supply is that they err on the side of caution, and are often a patronage job. This means that on the weekend some politicians ner do well relative has got the crummy shift, and will seriously dose the water.
As far as any metals are concerned, any water conditioner that states that it deals with heavy metals contains ethylenediaminetetraacetic acid, or EDTA. This is a chelating agent that binds heavy metals, removing them from any availability to your fish. Obviously a water treatment with EDTA is not one you want to use if you are using copper as a treatment. I use one with EDTA due to my water supply, which I could write a short story on, Chicago has a river that they got to switch directions 100 odd years ago, but it sometimes goes the wrong direction.
In a 30 year old house any copper in the plumbing is sealed long ago, nothing to worry about unless you have had recent plumbing work where they used copper. Your plumbing may be galvanized steel, all of mine is, as is common in older houses in my area. A quick look around unfinished areas should tell the story on that.
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