Sudden Die Off...

Things had been going really well in my new tank.  2.5 weeks old, kept my water parameters in check.  Then in the past 24 hours I have lost 6 fish.  Hoping for some help trouble shooting this.
 
 
Tank size: 75 gallons
pH: 8
ammonia: 0
nitrite: .25
nitrate: 2
kH: Unknown
gH: I believe 345
tank temp: 78

Fish Symptoms (include full description including lesion, color, location, fish behavior): The four I lost over night, showed no abnormal behavior yesterday evening.  The two I lost today slowly faded away, but were normal yesterday

Volume and Frequency of water changes:  25% every three days, I did a 50% today

Chemical Additives or Media in your tank:  General declor. API Leaf Zone added weekly very carefully measured amounts, API Stress Zyme+  This is added weekly, but yesterday AM was the first time I had used this brand.  I was out and bought it yesterday.
 
I have a Rena XP with the standard media in it, bio stars, sponge, white one..(can't think of what it is lol) and carbon

Tank inhabitants: 3 platies, 10 Minor tetras, 5 glow light tetra (ALL DEAD) 3 bala sharks (1 DEAD, 1 I don't have much hope for) 2 Black skirt tetras

Recent additions to your tank (living or decoration):  10 or so live plants. Myrio was newest addition.  Pea gravel and Floramax plant substrate 

Exposure to chemicals: None

 
Thank you!

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I will try to get back to you on you how to stop your fish die off as soon as possible but what I can tell you right now to to get those balas out of there, they need at least a 6 foot, 150G tank and best kept with at least 3

will look into it myself but all I can say for now is to individually look up water conditions ( pH,alkalinity,hardness,ect...) for your fish and to make sure its a match.  

pH is high for tetras, and balas I can tell you right off firsthand experience can be very sensitive when young so I would suggest lowing the pH to 7.5 and keep me updated

keep up with water changes and feed your fish less to lower your nitrites to 0 they should always be 0. sorry about making a bunch of separate post its late and my brains not working

nitrates should be good at 20

so get nitrites to 0 and pH to 7.5 and your should be good.

Well after lurking on some of your other threads, I can only come to the conclusion that your water changes are not enough to cope with your fish in cycle my dear.
I would suggest maybe 25-50% a day depending on your parameters.
 
 

rainbowsharkman said:

I will try to get back to you on you how to stop your fish die off as soon as possible but what I can tell you right now to to get those balas out of there, they need at least a 6 foot, 150G tank and best kept with at least 3

will look into it myself but all I can say for now is to individually look up water conditions ( pH,alkalinity,hardness,ect...) for your fish and to make sure its a match.  

pH is high for tetras, and balas I can tell you right off firsthand experience can be very sensitive when young so I would suggest lowing the pH to 7.5 and keep me updated

keep up with water changes and feed your fish less to lower your nitrites to 0 they should always be 0. sorry about making a bunch of separate post its late and my brains not working

nitrates should be good at 20

so get nitrites to 0 and pH to 7.5 and your should be good.

Click to expand...

 
You've already stated you have intentions to get a bigger tank later on so that's fine, even though ideally the Balas would prefer to be in a shoaling group of 6+
The pH is quite high and as far as I am aware, the higher the pH the stronger the toxicity of Ammonia and Nitrite is so more frequent water changes is a must.
 
Could always try picking up some Seachem Prime if you can source some? I've heard very good things about it, it's a dechlorinator but it also makes the Ammonia and Nitrite harmless (for I believe 48 hours after the doseage?) while allowing the filter to still process it during the cycle. Sounds like a win win really, keeps the fish safe but allows you to do the fish in cycle.
 
Don't go messing around with the pH either, I would say big swings in pH are more stressful and probably more deadly to fish than your current Ammonia and Nitrite and it'll just go back up when you do water changes (not to mention the cost of buffering powders or liquids and the potential of pH crashes!).