I guess the only way to find out if the sand filter can handle or not by doing a test run on it for a week and if the water parameter is still good that mean the media is able to support the stock.
Thats the beauty of K1. You don't have to run tests to see if you have enough. There is actually calculations you can do to find out how much you need.
The first thing you will need to know is that K1 needs about 1LPM of air per liter of K1. This flow rate allows it to fluidize properly. How and where to place the air depends on shape of container the K1 is in.
1 liter of K1 can handle 5 grams of 40% protein waste a day(with ease).
Feeding rates are typically around 3% of a fish's body weight.
A typical fish weighs around 10 grams per cubic inch.
Do the math and you will know exactly how much K1 you need to support your stocks feeding levels.
With the amount you get, double the number to allow for a good safety factor, and extra ammonia that will be produced threw respiration of your fish, and other ammonia sources.