Hi Jesse. The bubbles you saw after putting the acid,its indication that some pieces of your gravel contain carbonates.The carbonate problem can be cause of the source water too.
Some areas have high concentrations of carbonates and as a result you see the pH in your system up again, within hours after using the acid.Any acid you add to the system will consume the carbonates and dissipate as it does. In some hours later the acid will be so weak in the water and the pH will be back to the previous high levels.
If you want to keep the pH at low levels,the amount of acid you have to put is so big that can cause the death to your fishes.
If you have already fishes in your system,you are not supposed to drop the pH more than 0.2 to 0.3 per day and for sure you have to avoid the bouncing of the pH up and down.
In that situation you are better off just treating any top up water with acid to a pH in the low 6′s for 24hrs prior to adding to your main system, over time this will reduce the amount of carbonates in the system water and the pH should begin to drop naturally as acid is constantly produced by the nitrification process.
If there is no fish to the system you can put acid on a regular basis and check it till it will be down around the low to mid 6′s. Do this until you find you are having to add less and less acid each time to have the same effect. This process will have consumed a good portion of the carbonates in the water.
Take care to don t drop the pH so low during cycling! The bacteria prefer the pH to be little be high and require some carbonates too. If after treating the water with acid for some days you manage to have the pH at 7.0 then everything is right.
Another alternative,is to leave the pH at 8.2 and just treat your top-up water, then see what happens over the next few months with nitrification and top-ups. However, I’ve had two AP systems where the pH has simply refused to drop below mid 7’s… one current system is over 18 months old and still sits around 7.4-7.6… even with treating top-up water with acid first… but it used to be even higher before I started doing that… around 8.0
You have to decide alone which approach you want to follow, i don t want to convince you in any direction, just want to give some ideas of dealing with the problem.
Wish you good luck with your aquaponics system!