Fish are Pets, too, right?!? - Urgent Help Needed-

Does anyone on this thread have fish tank knowledge???

If so, I have an urgent question for you....
I have a 10-gallon hex tank with a couple of tetras, two frogs, some snails, and added 3 angelfish 3 weeks ago.

I changed the water yesterday. Before changing it I checked the water levels with my "Quick Dip 6-in-1 Stick" and all looked fine. I did it again after changing and all looked fine. This morning I woke up and one of my angels was dead and a second is not looking like he'll survive to mid-day. I checked the water levels and it looks like the alkalinity is high (like around 180 - color blue on the stick). The ph is also in the alkaline levels (somewhere around 7.8).

Anyone know how I can fix this to save the other angel? The other creatures in the tank all seem fine. Its just the angels that are a bit more delicate.