In America they mainly use chloramine in tapwater to kill bugs. This is a chlorine and an ammonia joined together. All dechlorinators split chloramine back to chlorine and ammonia then remove the chlorine but they leave the ammonia part still in the water. So they add another ingredient to the dechlorinator which detoxifies ammonia for up to 24 hours. During this 24 hours the filter bacteria eat the ammonia so that by the time the detoxification wears off, all the ammonia has gone. Since a lot of dechlorinators are made in the US, they contain the detoxifier for the American market.
In the UK we mainly have chlorine in tapwater not chloramine, though some water companies are starting to use it. With chlorine, the dechlorinator removes that but there is no ammonia to detoxify so if your water company uses chlorine you don't need that second ingredient. My water company uses chlorine which is why I use a dechlorinator that doesn't detoxify ammonia as I don't need it.
A lot of dechlorinators also contain aloe vera, and the one I use doesn't - I don't want aloe vera in my tanks, just a personal preference.
So if you have chlorine in your tapwater you can use dechlorinators that do contain the ammonia detoxifier or you can use one that doesn't. If you have chloramine, you need one that does detoxify ammonia. Your water company's website might say what they use or you could always ring/email them to find out.
You often get very fine particles in your tank which make the water look a bit misty. These are so small that they go right through a sponge. Filter wool is fine enough to catch most of these tiny particles.
This kind of thing. You cut it up to the size that fits your filter then lay a piece on top of the media in the last basket with an external. It isn't very thick so it doesn't take much room. Filter wool is not a very good home for bacteria, it clogs quickly if you have messy fish and it goes into holes and shapeless after a couple of washes. Because it doesn't hold many bacteria it is safe to change it frequently.