Welcome to our forum OwnenB86.
"I've recently learned that the goldfish can grow up to 10" and need about 60l each" Don't worry so much about your moor, it will only grow to maybe 4 inches long, not 10 inches. The comet will indeed grow quite large but that could take many months to happen. As of right now you are doing fine. At 200 l/hr on a 29 litre tank, you have plenty of flow. The sandstone does not have the appearance of rocks that affect water quality, but why not run a test on your water? If the rock is affecting your pH, you will be able to measure the change over time.
I'm afraid that fluttermoth has accepted a rather common myth that fish can only grow so large based on tank size. It is simply not true. Fish will continue to grow as long as their water is kept pristine and they have plenty of food. Unfortunately maintaining pristine water conditions becomes far more difficult when you have too large a fish load in a tank. In commercial circumstances, the huge filter systems they use and the amount of water exchange they do means that hundreds of 2 inch long fish can easily be maintained and will grow rapidly in a rather small container, by hobbyist standards, maybe even in a 50 gallon container. An example cited by a commercial breeder who recently lectured at a national convention that I attended gave an example of some catfish that he keeps in his central filtration system. The total volume of the water system ran to many thousands of gallons but the container these 3 foot long catfish live in is only about 6 feet long. By his statements, they each need to move aside to allow the next fish to feed, they are packed together that tightly. Even so they have grown rapidly to their present size. Mere size is not the controlling factor, water quality for those crowded catfish, is always great and they are fed culls from selective breeding along with their normal pellet food so their diet is quite rich. They thrive and prosper where many of us would expect them to die from the crowding.
Tank size is not the main factor for stunting but maintaining water quality while feeding very well can be a challenge unless you are set up to do it. That is why we often see stunting in small tanks, not the tank size itself.
