the only stupid question is the one that isnt asked.
just syphon out 25-50% of your tank water, using the suction to hoover up any poo and other solids from your tank too and replace with fresh water from the tap. its also advisable to give your gravel a quick vac while your doing a waterchange, you can buy them from pets at home. the non return valve on them means getting a syphon going is a breeze and the plastic vac fits perfectly onto a regular garden hose if you dont want to caryy buckets of fish water around. the waste water is also great for grass and plants. always use a good dechlorinator, i use seachem prime.
im pretty sure that none of the water boards in the uk use bog standard chlorine anymore. they all use chloramine which needs to be broken down by a dechlorinator that states it does chloramine on the label. seachem prime is also good should you have a nitrite/ammonia spike.
