Really depends on temperature as well as oxygen level. Clean aerated water is a must. The other thing to consider is feeding the minnows and keeping the water clean. I keep minnows in an 80 gallon aquarium with a power filter system and run a DE filter every couple days. My water stays 55 - 65 degrees. I buy 5# of minnows at a time, replensih when they get time consuming to net, Conservatively. I would guess I keep 3-7 thousand minnows year round.

As far as being too slick, the slime coating is needed to maintain optimal health. However, the chemicals you added may have been done to an extreme. I do not use chemicals as I am using well water that is filtered through a charcoal filter before adding to my tank. I do add 1 teaspoon of sea salt for every 10 gallons of water added.

Keeping water clean and fresh is the hardest part. changing 15 - 20 percent of the water weekly helps to remove toxins from waste decomposition. The nice filter system helps a whole lot but the gravel bed needs weekly maintenance simply siphon out all the waste and add fresh water. After several times you get very efficient and water additions are simple.