The cleaning routine recommended nowadays is to do a 50% water change and gravel clean every week. And by cleaning the gravel I mean pushing the siphon tube right down into to the gravel to suck up all the muck (fish poo, uneaten food etc) that gets down between the particles.
With fish that are sick, a daily water change often helps.
As for dechlorinator, that depends on how you refill the tank.
If you use a bucket, add dechlorinator to each bucketful of water at the dose rate for the volume of the bucket. For instance, my dechlorinator uses 1 drop per 4.75 litres. I put 2 drops of dechlorinator in the bucket then run in water up to the 9.5 litre mark, then empty that into the tank.
If you use a hose pipe, work out how mcuh water you've removed, add enough to dechlorinator to treat that amount of water straight into the tank, then refill.
If you have a combi boiler you can use hot tap water to warm the new water to roughly the same temperature as the tank. if you have a hot water cylinder with a header tank in the attic, boil a kettle of water and use that to get the temp right. It's because of what might be in the header tank that you can't use the hot tap when you have a hot water cylinder. You don't need to measure the temp with a thermometer, feeling old water and new water with your hand will get it close enough.
Can you tell us what model of filter you have? That'll make it easier to say about carbon granules. In my case I have a small cheap internal filter and I made a bag from from some net fabric. I just get the filter out of the cupboard, pour granules into the bag, wash it thoroughly under the tap as carbon granules are very dusty, then put the bag into the filter and the filter into the tank.
But you can also use the carbon pads you mention, it's just that they are expensive as you have found. Since the carbon has to be thrown away once the old medication is removed, the cheaper the better. And depending on what your filter is like, you can probably use a bag of granules in place of a carbon pad.
If you already have a carbon pad in the filter, remove it before adding medication. Although carbon gets full quite quickly, even if carbon has been in the filter for months there is the theoretical possibility that the medication will be more attracted to the carbon than what's already there, so it will push the current stuff of and stick to the carbon instead. And medication stuck to carbon can't do its job.