Hi and welcome!
Unfortunately, stability contains the wrong kind of bacteria, so you have to keep adding it otherwise your tank will lose its cycle. The only product that seems to really cycle a tank almost "instantly" is called Tetra Safe Start.
Also, if you change your filters it will kill the cycle as well. The only way to prevent this is to keep the same floss that you've been using, and add it to the new filter. OR start running both filters and wait until the new one becomes "seeded".
It is also important to know your other readings,
nitrite and
nitrate. Without those it is impossible to know if your tank is cycled.
Also, cycling with fish is slower than doing the "fishless cycle" which consists in adding ammonia by yourself. With fish in the tank you should be doing water changes every day until the cycle completes to prevent the fish from suffering ammonia poisoning (which is very painful and sometimes lethal) ... unless you find Tetra Safe Start.
As far as the stocking goes, in a tank that big you have a lot of options, Just make sure to research all the fish you like, their requirements and necessities, before you go out and buy them.
Good Luck and let us know how it goes.
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