Fish nervious and not eating well

My fish were very active and eating very well, since about 2 days ago
they are hiding and not eating well and some, not at all.
These are angelfish, neon corys, 11 otos, 12 cardinal tetras.
Six (6) cardinal tetras have died in the las 3 days and think they are the ones with very drawn(empty) stomachs.
125 gal/100 gal net started 9/17, planted tank 10/11.

The significant change that I notice is PH now 6.4 from 6.6.
Have been dosing the same for 6 weeks except for a small increase in Plantex
and new dosing of Epson salt , 3.0g, 2x week.
Temp is about 80-82 F and steady.
There is green algae. Today I changed 50% water and they came alive for a while in the fresh water but reverted to the above behavior.

Any ideas?

tks

 

Yes I used dechlorinator (fish's behavior improved after water change).
Ammonia is -0-
CO2 - 35PPM (KH 53.7 , PH 6.4)

I had PH at 6.6 for a while but increased CO2 (lowered PH) to fight algae.
I am going to increase PH to 6.6 as this seems to be the problem.

Any other ideas?

tks

 

I am measuring my CO2 with my aquarium water, my KH and PH readings.
I don't know what a drop check is. Could you point me where to find out about it and which one you use, etc?
My PH has been 6.4 (35ppm C02 per table) brought it down today to 6.6 since fish were ok at that PH level ( so now CO2 should be at 23ppm)
My KH ..53.7ppm (3 degrees K)
My Nitrate today 20ppm (I just dosed fertilizers including KNO3)
My fish were happy Oct. 27 for sure, changed 50% water and added, for the first time: 1/4 tsp MGSO4(EPSON SALT) and
30ml of Excel. Obviously I overdosed. Have stopped using excel until I understand it.
At this point some fish died off.

Today I have installed a 25 Watt UV sterilizer to help with diatoms and green algae. BBA has started to appear now.

Your opinions would be appreciated

 

Excel Seachem response.

This is Seachem's response to my e-mail on the adverse effects to my fish using Excel:

Hello,
Flourish Excel is a reducing agent and has the capability
to remove some oxygen from the water. If you already have
a low oxygen content in the tank and dose Excel, this
could potentially have an impact on very sensitive fish.
However, this is a rare situation. We have these types of
fish in our tanks here and we dose Flourish Excel on a
daily basis with no problem. If you are using another
reducing agent along with the Flourish Excel, it is
possible that the fish are becoming stressed due to lack
of oxygen. Most water conditioners are reducing agents.
You may just need to add the Excel on days when you do not
add any other supplements and add less than the
recommended dose. As I said, this is not a typical
reaction with Flourish Excel. It is most likely not the
Flourish Excel alone, but a combination of factors.
Tech Support
10201
Tech Support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Seachem Laboratories, Inc.
1000 Seachem Drive, Madison, GA 30650
888-SEACHEM Fax 706-343-6070

What other reducing agents are there in our daly fertilizing?