Illness/Sickness: Sudden rise in pH?

Hi everyone!
On Sunday afternoon, I did a much needed & thorough cleaning of my axolotl's tank. Squeezed excess debris (in tank water) from the sponges of my filter, cleaned out the filter piping, vacuumed out some waste (my tank is bare bottomed) and performed about a 40-50% water change with Seachem-treated tap water. I've done all of this many times before without issue.
By the next morning, I noticed my lil dude was acting out of character (lethargic, kinda floating, "deer in the headlights" kind of behavior).
Upon testing the water, I found my pH to be very high, around 8.4-8.6. Nitrites are slightly elevated and ammonia is at zero. I've never had or at least been aware of pH issues with my water in the past, even when establishing my tank. I did a partial water change at that time, though I'm not sure what good it did, because I found my tap water pH to be even higher. I've tested it before without these results, I think it was around 8 the one time I tested.
So my questions are:

• What can I do to lower pH in the short term? I'm going to buy a few gallons of spring water after work today, but are there more effective solutions?
• Are water changes going to even help lower the pH?
• Can certain events cause a sudden change in tap water pH? I live near Houston, TX, and we've experienced some severe flooding over the last several days. Could that play a role?

I'm doubly worried because I use Seachem-treated tap water for my terrestrial sals too. D:
Thanks in advance!!
Harly

What are the test results for your NitrAte? For a cycled tank you should be looking for 0ppm Ammonia 0ppm Nitrite 20-40ppm NitrAte. I have a feeling you might have affected your cycle with the clean up you have done, or something is interfering with your test results.

I'm not that knowledgeable when it comes to PH, so perhaps someone else with more experience can advise.

Enough bottled still spring water to replace most the water in your tank could lower the PH.

Have you added anything new to your tank recently?

Is your water source from a well?

This article may give some information in regards to what PH is:
Axolotls - Requirements & Water Conditions in Captivity