Without a thermostat to control the burner you have to watch and manually control the flame. Hopefully you have a regulator with a quick adjust knob on it. I sit down right by that regulator and watch the temp and run that burner up and down to maintain the right temp. Typically I am going to bring the oil to temp running the burner at something like half flame...that helps to prevent overshoot. Then as I load in fish I am going to typically go to full flame to "catch the load" so to speak. Typically I am going to be able to stay at full flame for a minute or two...then as the moisture steams off the fish and it warms up the load is going to drop again and I have to turn down the flame to again keep from overshooting. How cold the fish is and how much fish you put in in relationship to the pot size and oil quantity and the burner size...determine how active your gonna be in the process.You will get good at anticipating exactly what's going to happen and can react early.

I have serviced commercial restaurant equipment for forty years. This is basically what they do automatically...throttle the flame up and down from start to finish. The closer you can keep your oil at one temp over the entire fry the better the quality . You can start the oil at 375 and say when loaded it may temporarily drop to 300 and come back and it won't be the end of the world. The fish will be edible....but the best quality comes from small variations in temp of say 20 to 30 degrees or so.