First, on the "clarifier," do not use this if you have fish in the tank. There is no need for clarifiers; if the water becomes cloudy or hazy, there is always a reason and you should work to discern the cause and then deal with that (if needed). The water in a new tank will almost always be a bit hazy or cloudy at least for a short time (hours to days) because of the bacteria establishing, and sometimes from sediment in the water from the substrate or the tap water itself. If this occurs after the tank is established, then there is likely a cause you need to deal with. Partial cloudiness at a water change is normal and not a problem. Most clarifiers work by binding together microscopic particulate matter into larger particles that can more easily be caught in the filter media; unfortunately these products also bind the fish's gills. Second, these are chemicals that being in the water will end up inside the fish; I'll have more on this below.
To the conditioner. This should only be added in the amount necessary for the volume of fresh water. Example, in a 10 gallon tank, if you remove/siphon out say two buckets of water that hold 2.5 gallons each, you will obviously be adding 5 gallons of fresh water. The fresh water only should be conditioned, in the bucket if using a bucket as you likely will be doing with a small tank like a 10g.
Every manufacturer will say that overdosing water conditioner is harmless to fish. This is false. Any chemical substance added to the water the fish live in will get inside the fish and into the bloodstream and internal organs. While most will not kill the fish outright, they are not beneficial to the fish in the sense of being in the fish's bloodstream and the less of these the better will be the fish.
Anbother thing to look for is additives to the conditioner. Some have aloe vera that they say is "safe" but this is no longer accepted; there is evidence that over time aloe vera in the water will damage fish's gills. One conditioner has valium; now, what possible benefit could that be to fish? Basic conditioners are all you need, those that detoxify chlorine (and chloramine if you have this in your water), and detoxification of heavy metals is usually included. Anything beyond this may not be necessary, and only add yet more chemicals. I have been using the API Tap Water Conditioner for ten years now. It is the most concentrated conditioner so you use very little, one drop dechlorinates a gallon of water (use 2 drops if you have chloramine in the tap water).
