most test kits are for ammonia, nitrite, nitrate, pH, KH, phosphorus. If any of these are out of whack, it will usually affect your fish. And depending on how much water movement+fish load+water temp, you may also have an O2 depletion but you'd know that if you see your fish at the surface gasping, which isn't a behaviour you're reporting. Remember; LIQUID test kit under a year new/unopened so you don't get false numbers. And if you do and report back, have the NUMBERS, not 'the params are all normal/within spec' as this doesn't tell us as much as the numbers. MOST people with problems with their fish are from water params that are out of whack or some chem got in by mistake (sometimes NOT by mistake ala algaecides--which we DON'T recommend you use--there's better ways!) Let us know and we'll see if anything is obvious or further testing. Include fish type/size/quantity and pond gallonage/dimensions incl depth, whether you have waterfall/aerator and basics of your filtration. A lot can be figured out by that info! And some indepth watching of the fish behavior; I'm a firm believer that if you watch your fish regularly, you'll know something's wrong well before bad stuff takes it's toll, though looks like that might be a bit late now, sorry to say!
edit; do you add dechlor when you add water? Do you need to (i.e. are you on a well?)?
