how can the fish tank PH drop rapidly that killed off all 6 of my koi fishes?

Question

There are 6 Koi fishes, ranging from 3 inches to 11 inches.
There are algae in the 50 gallon fish tank.
The gravel is about 1 1/2inch thick.
is this part of the problem?

Answer 1

Need a little more info from you to be able to help...

What is the hardness of the tank water? Both general and carbonate if possible.

What was the pH before the problem and after?

Also what had been done in the way of maintenance in the last few weeks?

Any special decor or other items in the tank?

Those will help us come up with an answer.

MM

Answer 2

Were you planing on putting the fish in a backyard pond? Even in a 50 gal. tank, this reads as if the fish may have been too crowded. Did you have an aerator going nonstop? What was the pH before the problem arose? Have you been testing the pH levels each week?
When the pH drops below pH 5.5, acidosis occurs. At low pH the fish begin darting around the tank, breathing rapidly, and/or may jump at any movement near the tank. Death can occur very quickly if the drop is rapid (pH shock) even when the pH does not reach 5.5. When the pH drops slowly, the same symptoms will gradually emerge; gasping due to lowered oxygen carrying capacity of the blood, heavy slime production which reduces oxygen exchange over the gills, the precipitation of any metals in the water into the gills, fish darting into the side of the tank. Metals are very toxic at low pH. Gills will be dark red to almost brownish.

Pond fish are more tolerant of pH fluctuations than tank fish. Outdoors, submerged plants/algae make carbon dioxide during the night. CO2 dissolves in water and makes an acid that can drive the pH down into the 6.5 range. In the morning the plants/algae use the CO2 and the pH rises. At night, algae filled or polluted ponds/lakes can also use up the dissolved oxygen. This means the fish are either gasping at the surface or found dead in the morning. The largest fish die first. Immediately increase/add aeration whenever a fish is seen gasping at the surface. This is one reason that pumps and waterfalls are never turned off, especially not at night.

Dissolved CO2 cannot drop the pH to 5.5. But decaying organic matter in the tank or pond will drop the pH that far. Feces and rotting food in gravel in the bottom of tanks, dead leaves etc. in the bottom of ponds decay without oxygen. This leads to incomplete breakdown and the accumulation of organic acids, fermentation products, including toxic gases.

Answer 3

You don't indicate how long the tank has been running with these fish in it. A general rule for fresh water fish is 3 gallons for every inch of fish in the tank. Also, the tank has to do something called cycling. This is a process where beneficial bacteria multiply to consume and convert the harmful excretions of the fish. Fish give off ammonia (NH3) which will lower pH quickly. There are bacteria (actually aerobic bacteria) which consume the ammonia and give off nitrite (NO2) Nitrite is not as bad as ammonia, but another bacteria grows quickly and changes this nitrite to nitrate (NO3) which freshwater fish tolerate. Even after the water has cycled and there are lots of bacteria changing the ammonia to nitrate, it is a good idea to change 10% of the water every week or so and put in fresh water.

Answer 4

kio (goldfish) are not easy to keep right .. why do you say the ph dropped out of the tank killing the fish , i am more inclined to believe that the water fouled up , and got ammonia, and nitrate poisening instead .. the gravel on the bottom isnt a good idea with gold fish since they poop about 2/3rd's more then regular fish, and daily vacumming of that up is nessecary to keep the water in good condition. they need alot more flitration then regular fish because of this . go to petsmart, and look at thier gold fish tanks , and touch the sides of it . (its a chilled water) with little or no decor on the bottom for exactly this reason. with gold fish stock like you have big to small i would change 10% of the water every other day .. if they all died in the tank , i would nuked the tank with a gallon of hydrogen peroxied , and replace the filters pads, and change 3/4 of the water after runnning the tank with h2o2 for 24 hours with filter running , then vacumm it again to half full , and refill with water, and treament, and (if there is no fish) . this will reset the tank ,and is the start of a nice tropical tank . cover the bottom of the tank with plants, and get a heater in there , and go tropical freshwater fish instead .
read up on tank cycles, and how planted tanks effect this ..