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It seems that in Buttons.java, if you remove the

tank.repaint() and repaint() parts, then the weird glitchy painting goes away, though it won't update. So the error is something to do with...

No, but I think I noticed something interesting.

if (e.getSource() == fill) {
tank.fillingWater();
tank.repaint();
}
else if...

I fixed that. It would seem that it only really dumps when you're either at the top, or even worse, more than overflowed.

Perhaps first you should try and make it so that it can't go higher...

Is it compiling?

I'm getting these errors:

DropButtons.java:14: type javax.swing.JComboBox does not take parameters
JComboBox<?> fillingRate = new JComboBox<Object>(rates);
...

Where are you having a hard time?