Hello; You wrote that you turned up the heaters as far as they would go. It occured to me that this may eventually lead to overheating. The adjustment knob on heaters that I have are to set the thermostat near a desired temperature. My understanding is that a 300 watt heater operates at full capacity of 300 watts any time the thremostat senses a temperature drop and turns the unit on. Mine have a bimetal strip that moves in or out as the temperature changes. (Bimetal is two different types of metal snadwiched together. Each expands and contracts at a different rate causing the strip to bend one way as the temp rises and back the other as it drops.) There is a small contact point on the end of the bimetal strip that will eventually touch another contact point on a stationary strip as the temp drops. This closes the circuit and turns the heater on untill the rise in temp causes the bimetal strip to move back in the other direction, breaking the circuit and turning the heater off.
I have not used the submerged heaters yet (I have one acquired with a used tank, but have not examined it yet.) This description is for the heaters that hang on the rim of a tank. Your heaters may have a different type of thermostat, but I will be suprised if if the basic operation is very different.
When starting out with a cold tank, I put the heater in and allow a few minutes for it to equalize with the tank water. I plug it. If the indicator light is on, I rotate the adjustment knob counter clock wise untill it goes out. I then turn it clockwise until the indicator light stays on and steady. After some time the light will beging to flicker on and off every few minutes. Check the temp and if too low give it another clockwise twist untill the indicator is steady again. Repeat this untill the tank is at a desired temp.
Some one familiar with submerged heaters may be able to give you more correct information on those types.
