The flicker is due to the ballast in a different way. An Electronic ballast will have virtually no flicker while you see it where an electric will. An electric will also 'flicker start' the tube. That is what degrades them so quickly.
The ballast is basically a large 'resistor' type object. It protects the tube from power surges. Therefore igf you put the wrong size ballast with the wrong size tube you can have problems in that the light doesn't start or it is underpower or overpowered.
LEDs are similar in that we use a current controller on them.
If I use a 12V adaptor to run 12V worth of LEDs you may think 'great its perfect' but there are spikes and troughs in current just as there are with most things of this nature. A spike of 5% could be the difference between an LED running at full pelt and actually melting!!! Therefore the current controller 'limits' the power. It makes sure the power is constant. Therefore making sure that you aren't replacing the LEDs frequently. A protection if oyu like.
Now think of the ballast in the same way. If you have a ballast rated at 15W then it protects the light within a tolerance level of say 5% (example and not to be taken as accurate.) That means although the tube may get 5% more occasionally but not enough to burn the tube out.
This is why a 20W tube on an 18W ballast will only get 18W (give or take the tolerance)
Turn it the other way around and run a 20W tube on a 30W ballast and you are now at the level where yes the tube will work. It may last quite a long time but it could go 'pop' at any time if it receives more than it can handle.
If a light is flickering badly it more often than not indicates that either it is not receiving enough power, or it is nearly dead or the starter needs replacing or that the electrics have a problem. Overloading will not produce the flicker. Overloading will either have the light burning very very brightly or burnt out and not lit at all.
Flourescent lights aren't as delicate as LEDs. Many manufacturers use a 'close' ballast to the tube rating. The old arcadia type external ballasts clearly state 15-18W etc. They are for a range from and to. they will however be set to handle the lower range thus limiting the higher range. These are magnetic (electric) ballasts that need a small cylindrical starter to work.
Electronic ballasts are a different kettle of fish. There are some that can handle all sizes, there are others that are specific to a certain wattage but they are much more constant. They can control the peaks and troughs much better and when using these a good quality tube will degrade only 5% or so over the first 40% of its life which means 2-3 years at minimum.
By good quality tube I am not talking the ones that have the very expensive letters AQUARIUM printed on them. In most cases they are on a par or worse than a reasonable tube and much more expensive to boot. I am talking tubes that have good CRI. Aquarium tubes tend to be in the 70-90% colour rendition accuracy range which is about par with the ones you get (except budget ones) from the DIY store for a quarter of the price. There are lights out there with CRI between 90 and 99% that are still half the price of the ones with AQUARIUM printed on them.
However most people seem to think that for an aquarium you need aquarium tubes therefore they pay the money. In reality the difference is those 7 letters printed on the box and/or tube!!! Expensive Ink
AC