No it wont mess it up, CT's are added to remove the clorine from tap water. The only thing that you will notice is a very slight metalic taste, simular to the taste you get from canned beer/cider or L***R.
I only started to use them when i joined this forum. Now when i make a kit, i treat the 25ltr of water need to make the brew with a CT then take 2ltr out boil it and add that to the kit, then top up with the remaining water.
There is a post on here somewhere on how to get the best from your kits, when i get chance ill find it for you.
the following is someting one of our moderators has put together, i'm sure he'll be fine that I've posted it
How to get the best from your beer kit
First read the instructions that come with your kit.
Although the instructions supplied with many beer kits are adequate to produce acceptable beer there are things that can be easily done to improve the final quality of the beer.
Sanitation.
Whatever method of sanitation you choose be thorough and rinse your equipment well with cooled boiled water, to remove any trace of residual sanitiser.
The next is water quality.
Tap water is perfectly good to produce your beer from, however, most tap water contains chlorine and chloramines which can combine with the ingredients in your kit to give a medicinal or TCP like taste in your beer. The easiest way to counteract this is to treat the bulk of your brewing water with ½ of a crushed campden tablet per 25 litres. Simply add your water to your brewing bin and stir in the crushed campden tablet.
Take the required water to dissolve the malt extract from this treated water and heat separately.
Sugar.
If your kit calls for extra sugar to be added, there are better alternatives to household sugar, these are in order of increasing quality, Brewing sugar(glucose), beer kit enhancer ( a mix of glucose and dried malt extract) and finally extra malt extract.
These can all be used as a direct replacement for the sugar called for in the kit instructions.
Aeration
To give your yeast the best possible start in life it needs a good amount of oxygen to be dissolved in your beer. This is easily accomplished by a vigorous stirring or splashing the cold water into the hot water / malt extract mix, if you get a froth on top of your brew you will have sufficient oxygen dissolved into your beer.
Temperature
Fermentation temperature is also very important. Ferment to warm and you will produce some unpleasant tastes and odours, too cold and your yeast will stop working. A constant 18 to 22C is ideal.
Finally patience, donât rush! Give your beer 10 days in the fermenter and allow the yeast to do its job and then settle to the bottom giving you clear beer.
Whether you bottle or barrel your beer give it a month to mature, itâll be worth it.