Seachem Prime

Hmm, not sure.

In theory, if all you are doing is diluting the whatever-it-is in Prime, it should still work when you add it to the tank. But if your tap water contains ammonia, and the UK does allow a small amount, the whatever-it-is might be used up by the time it goes in your tank. Ammonia in tap water usually occurs where the water company uses chloramine as disinfectant rather than chlorine.

Don't mix up chlorine and chloride 

Chlorine is a gas, and is dissolved in water to disinfect it.

Chloride is a salt of hydrochloric acid. The salt of hydrochloric acid and sodium hydroxide is sodium chloride, better known as common salt, the stuff we add to food. But there are loads of other chlorides which could be in our tap water.


Chloramine is a chlorine and an ammonia joined together. I think it registers on both tests. Dechlorinators split chloramine into chlorine and ammonia and remove the chlorine bit. That's why lots of products also contain an ammonia detoxifier, to deal with the ammonia from the chloramine.

Wow I didn't know that 

So when we use dechlorinator in water containing chlorine we create ammonia in the water (I know create isn't the right word... but its late...).

What sort of levels are we talking about here? How long would it take for the ammonia level in a cycled tank to return to normal again after say a 50% water change?

Almost correct Matt, its only if the water contains Chloramine that you are left with ammonia
I dont know what sort of levels, I guess that is down to how hard the water company is dosing
I dont currently  have chloramine in my water but I dont suppose they will tell me if they swith over to stsrt using it so i have a dustbin of water stored which i dechlorinate but there is also an old internal filter in the bin so if ammonia does start turning up it will have a chance to get broken down in the week

Found this info helpful
SEACHEM PRIME & SAFE;

Prime contains complexed hydrosulfite salts which removes chlorine, chloramine and ammonia. Prime also converts ammonia into a safe, non-toxic form (ammonium) that is readily removed by the tank’s nitrifying bacteria.
Prime may be used during tank cycling to alleviate ammonia/nitrite toxicity.

How Prime Converts toxic ammonia (NH3) to non toxic ammonium (NH4):
The process that takes place between Prime and toxic ammonia is not a straight conversion from NH3 to NH4.
Prime contains a binding agent that works to temporarily convert toxic ammonia (NH3) into a non toxic imidium salt using complexed hydrosulfite salts to chemically convert the ions in ammonia into a non toxic imidium salt.
It is noteworthy that this is a temporary conversion and Prime will only bind the ammonia in this form for up to 48 hours before the ions are free to re-associate into toxic NH3.

Prime also detoxifies nitrite and nitrate, allowing nitrifying and denitrifying bacteria to more efficiently remove them. It will also detoxify any heavy metals found in the tap water at typical concentration levels.

The ingredients of Prime also promote the production and regeneration of the natural slime coat and are non-acidic and will not impact pH.

USE:
For removal of chlorine, chloramines, and the de-toxifying of ammonia, nitrites and nitrates. This product adds electrolytes that aid in natural slime coat generation and proper osmotic function which generally all this needed for fish stress.
This is my favorite conditioner to use in newer aquariums, aquariums with high ammonia, for water changes in tanks under medication treatment, or for use in areas where chloramines are in tap water. One reason I like this product is that is does what it claims!
Prime is also a great product to use if high ammonia or nitrites are encountered during cycling of a new aquarium while fish are present, as this product will NOT interfere with the important Autotrophic nitrifying aerobic bacteria (as my own studies as well as lab studies have shown).

Please note that fishless cycling is best, please read this article:
Aquarium Nitrogen Cycle; Methods for Cycling

Please note that when using Prime/Safe (and Amquel Plus) for neutralizing toxic ammonia (NH3) that these products work basically "instantly" by converting toxic ammonia ions into non toxic bio available ammonium (NH4) ions.
There is no residual effect, so new toxic NH3 will begin to accumulate after addition of Prime (or Amquel Plus) and the water change, however generally if you are carefully feeding and keeping the bio load as low as possible this should be adequate if water is changed and Prime is added every other day (until the high ammonia "Crisis" is over).

SeaChem Prime works for ammonia detoxifying by containing a binder which renders ammonia, nitrite, and nitrate non-toxic.
As noted at the beginning of this article, most dechlorinators operate through a chemical process known as reduction. In this process, toxic dissolved chlorine gas (Cl2) is converted into non-toxic chloride ions (Cl-). The reduction process also breaks the bonds between chlorine and nitrogen atoms in the chloramine molecule (NH2Cl), freeing the chlorine atoms and replacing them with hydrogen (H) to create ammonia (NH3).