one of my platies went to heaven...

ok so I have the 55gal tank I am at the middle of my 3rd week of cycling, and today before I went to work I noticed that one of my new platies was dead...just laying there upside down at the bottom of tank. I immediately took out all of the lava rock I had put in the tank 2-3 days ago. (I had soaked the lava rocks for 48 hours prior to introduction into the tank and also I only put in certain ones, making sure it was all lava rock and nothing else, such as metal traces etc.) after taking out the rocks I did a quick 8-10 gallon water change, as soon as I get home on lunch break I am going to have the water tested, as far as the last test everything was ok except the PH was a bit high (8.0) the only other thing I can think is that he just died cause thats what fish do, I only had him barely a week.... but what would you say are the main things I need to look out for in the next few days????
thanks, allan
p.s. what else can be done to insure the lava rocks are ok, I got mine from the same place my pal got his and his fish are all fine? any idea?

well....the culprit was indeed ammonia. when I checked the water I had
(and again I'm sorry for not having exact PPM PPL)
ammonia low to mid
nitrites mid
nitrates low
alkalinity ideal
hardness 350-400
PH 8.4* *this has gone up .4 from last test I did 2 days ago

now I did add some lava rock in aquarium for 2 days but have since taken it out since I noticed the platy (I did rinse them) and untill I feel better about it I will keep them out for a while.
other than that things I have added....
-an airstone (12") that has added A LOT of bubbly-action
-I had some of the BIO-ZYNE bacteria stuff, and I have had some success with it in the past with the 10 gal overcrowded nightmare I experienced a few months ago, and also something else that worked back then was the
-AmQuel the stuff has worked miracle as long as I keep doing PWC every 2 days or so.

I should have figured this would happen....I am very bad at overfeeding my fish sometimes and I know that is what created such a spike in ammonia
(just 2 days ago I had virtually none in the tank when I tested it)
besides using zeolite what else can help that you could consider with these kind of situations?
(I am still only 5 months old in the world of aquaria)
I don't like finding dead fish in my tank it makes my stomach hurt...

sorry cause I should know this but what is the threshold for ammonia, and nitrite, nitrate? (for example .25ppm of ammonia is the most i've read that you should allow tank to be at. I tested today and
ammonia - .6 ppm
nitrite low
nitrate high
ph 8.4
alkalinity- ideal
hardness 350-400 ppm
anyway I used the amquel which detoxifies the ammonia to a degree, I will do a PWC tonight, but fish are a lot more active today than they were yesterday thats for sure. also I put more gravel from other tank over.
anyway what are acceptable levels on ammonia and nitrite, nitrate???
thanks

well I did another water change yesterday (15-20%)
when I checked the water parameters they were pretty much the same as yesterday...I need to get the API water testers...
but one of the platies had some fry (last night I guess)
never seen fry before, they are kinda cute little guys....buggy-eyed tho...
I am going to do another PWC tonight, another thing that I am very happy about is the cloudy water is finally gone for the most part, it still is only 3 weeks into cycle today. the other fish seem happy and I catch them swimming in the bubble-stream I put in 2 days ago...
is it common for platies to just sit completely motionless? they will do that in various places throughout the tank, is this normal behaviour???