+1You know, with the fixture you have you could easily stock some low to medium light plants in there. The water is just hard enough to support them as long as you don't try valls. A bunch of crypts would go nicely and some real amazon swords.
Just thinking out loud... the tank looks great without but I really like plants... so I like to suggest them.
Additionally you killed off all the algae in the drift wood which in and of itself could cause the low pH.
and possible ammonia.
Test ammonia also.
And most importantly did you use latex type gloves with any kind of oil or chemicals on them? Did you change the filter media? Did you accidently have any toxin in the tub? Was the dog (with a flea collar) jumping around the tub as you rinsed? This can happen after introducing toxins in the tank not to mention from the water change itself.
You may need to do a water change which I never recommend except in the rare unique cases where accidental toxins may have been introduced. If you have another tank I would transfer the fish there until this settles down.