Fish die out

Question
QUESTION: My stickcatfish, harlequins, khuli loaches, balloon Mollies, golden loaches, black Mollies have all died out. my gouramis, discus, rummy nose tetra, angels, dojo loach, bala sharks and corydoras all seem fine. I tested the water in the tank and came up with the following results:    
ph: 6.0
High ph: 7.4
Ammonia: 4
Nitrate: 0
Nitrite: 0   
I do not know if the ammonia is to high? Could you help me out? I have a 125 gallon freshwater tank.  
Thanks

ANSWER: Peter,
If you could elaborate as to what type filtration you have, temp of the tank, how long you have had the tank running and what the last fish you added was.
Now, no suprise the mollies died out as they are really brackish water fish, high ph and salt.
The PH is about right for "wild:" caught discus, however tank raised do not require such a low PH.
Actually if your ph gets any lower you willnot be able to grow plants and your fish may well contract acidosis.
I am a bit unclear on the note "high" ph. I am not sure what that means if you could explain it may help us out.
The ammonia is a bit high and lack of nitrite and nitrate indicates that the biofilter is not established or not working properly.
The harlequins should have been ok in the conditions however they are a bit delicate and the stress may have done them in.
As for the loaches if could be any one of a number of things, if the PH was indeed 6 that would have been a factor.
Lets get a better picture of the system and how it is functioning and we can outline a game plan to get you back on track.
Mail me again when you have the info and we will get started.


---------- FOLLOW-UP ----------

QUESTION: ok....I have an external canister filter, the temperature of the water is 31.9 degrees Celsius, I have had the tank operational for 10 months and the last fish that went in were some platies. the high range pH is a test that came with my Aquarium Pharmaceuticals (API)Test kit.

Answer
Peter,
at that temp. which if I am correct translates to 89.42F is too hot for everything but the discus.
You could drop the temp. down to 82F or so, the discus will do alright and the other fish will be able to tolerate that temp.
The high range PH will not read PH at the level that you are at so the 7.4 means nothing, the regular PH test will measure somewhere in the area of 5.5 to 7.8. that is the one to go by.
I don;t think that you are in a crisis situation and with a few parameter changes you should see a big improvement.
A 25% water change would not hurt things either, and would assist in upping the PH, unless its 6.0 out of the tap. If so I would locate a spring add some spring water to the tank.
In lieu of that, an RO unit would strip the water and you could adjust it as needed.
If you have more questions, just ask.