seaweed extract like Maxicrop or seasol are common ways to get potassium and they can be foliar fed as well as adding some to the system.
Potassium bicarb is a good one for keeping pH up.
Lowering pH is more of an issue since you need to figure out why your pH is high before you will know if lowering it is really an option. muratic acid (hydrochloric acid) is an option as are many other acids but you probably want to avoid having to constantly add anything to your system to lower pH. Switching to rain water or RO water for topping up.
A high pH (up to say 8.2) at the start is usually not a big problem as it will come down on it's own eventually but if you are constantly topping up with a lot of really hard water then you may have issues.
the PH of my tap water is 8.8 to start with is that going to be a problem?
With really high pH tap water you might want to look into using rain water or RO water. To figure out how necessary that might be you might want to run some experiments with a bucket or barrel of your tap water and some acid. You want to see how much buffer is in that 8.8 water to see how much of a problem it will really be. If it is simply alkali but a single small dose of acid will bring the pH down AND keep it down then there isn't too much buffer there and you might be able to cope with it just using a little acid to adjust your top up water before using it in a system.
However, more likely is you will add acid to the water and the pH will come down but the next day when you check the pH will be back up and it will take many doses to use up the buffer therefore liberating too much calcium into the water which might also cause you issues with potassium in the future. This is kinda the case with my well water so I've taken to collecting rain water for most of my system top up purposes at least in my two smaller systems. With the rain water I actually need to add buffers to my system to keep the pH from dropping too low so I can alternate between potassium bicarbonate and calcium carbonate to also keep the potassium and calcium levels good.
Yes there are many pH threads.
Beware of Hydroponic pH adjusters and check the ingredients before using them. Some are just fine like phosphoric acid but others have lots of ingredients including ammonia and can cause a terrible ammonia spike which is bad for fish if they are already in the system.
I've used pool acid before (sulfuric acid) but that isn't something you really want to be using huge amounts of regularly.
Others have used muratic acid (hydrochloric acid) which can be found at Ace Hardware in my area.
I don't really recommend adding acid directly to an AP system and I wouldn't recommend using it all the time. If your source water is so hard that you need to use acid all the time you may be better off collecting rain or using an RO filter or something or even a water softener as long as it doesn't add too much salt to the water.
If your media is causing the high pH, don't bother with acid as it won't work and will cause more problems than it can possibly fix.
Vinegar might work in a pinch but it breaks down and isn't long lasting. (It may also have some antibacterial properties that are not desirable when cycling a system.)
I don't recommend citric acid since the antibacterial action of it is stronger and can really inhibit bacterial cycling.