Obviously, it was a good thing you were there… And, when in doubt a water change is always a good idea.
The milk would have to be thick enough to clog fish gills to be an immediate threat. My guess would be 8-gallons plus of milk in 63-gallons of water (12-15% milk, assuming a standard 75-gallon tank setup) would constitute an immediate threat to the fish. Potential changes in pH and Osmotic pressures present a longer term threat at higher dosages of milk.
Arguably, the milk would be beneficial to the plants.
Assuming for the moment the total 2% milk added was about a cup, call it 240-ml to 63-gallons of water, call it 240-liters (kinda makes the arithmetic easy) the kids added about
- 51-ppm C₆H₁₂O₆,
- 34.8-ppm protein
- 20-ppm fat,
- 6.2-ppm K⁺,
- 1.2-ppm Ca⁺⁺,
- 2.9-ppm PO4⁻,
- 0.42-ppm Na⁺,
- Bunch of trace stuff, none bad
While not devastating, the sugar, fat and proteins are the problem, the little beasties would love the sugar the protein and fats will be broken down, if you have excess capacity, your biological filtration should produce something in excess of 55-ppm Nitrates, though depending on your plants and water pH a good bit may be consumed as ammonium.
- As pH (and temperature), increase the risk of ammonia poisoning, increases.
The real problem is there is not likely to be that much biologic filtration reserve. The result becomes higher levels of total organic carbon (TOC); often this is measured as chemical oxygen demand (COD) or biological oxygen demand (BOD), though technically not the same, for our purposes pretty much interchangeable.
As a down and dirty check on this, I recruited a couple of sunset platys, Xiphophorus maculatus, a few pond snails and some water wisteria, Hygrophila difformis for a little time in a 7% 2% milk in solution with aquarium water. Two hours in the solution and now in a quarantine tank no one seems the worst for wear, but I will keep an eye on them for a couple of weeks to see.