PH spike...wth?

[Botanica]

Hi everyone...ok so last night we did the weekly pwc (20%) on the 240 gallon. Levels before the water change were
Ammonia <.25 (it wasn't yellow...but it wasn't green. We had an upside down cat missing...so I was suspect that he was causing the small ammonia increase)
Nitrite 0
Nitrate 10
pH 8
Temp 78
The pH in this tank is always 8...every time I have tested it. The substrate is coral...so I understand it buffers and that's why it's always 8.
Today I tested like I usually do the morning after a pwc and now I'm confused...
Ammonia <.25 (still...? We found the cat...ugh)
Nitrite 0
Nitrate 20
pH 8.4-8.6!!!
Do I need to worry about this? Will it settle back down? The tank isn't clear yet...but it's usually a bit hazy after the pwc...should settle down by this evening.
Any suggestions?

 

[aqua_chem]

How old is this tank? Is your tank sufficiently established?

Barring something obvious, large water changes can cause false positive ammonia tests as a result of the dechlorination process. What dechlorinators break down chloramines in chloride and ammonia, both of which are subsequently bound by the dechlorinator. Ammonia test kits can 'steal' the ammonia from the dechlorinator, especially if it is allowed to sit longer than 5 mins before being read, leading to a false positive.

I would continue testing it, and if it ammonia keeps appearing than you can assume it's not this.

As far as the pH goes... it very well might be the coral, or it might be something in your water. Sometimes water needs to offgas a bit before reaching a stable pH.

 

[Botanica]

The tank is established...up and running with good test results for 4 months. I haven't added any fish in over 5 weeks and everything has seemed okay...besides the missing catfish. I don't know why he died....:-(.
I wasn't as worried as much about the trace ammonia results as the pH spike. It has been a stable 8 every time. If its nothing to worry about...I'll just wait it out. Thanks!!