After that you’ll get 1800 coins per quest, which helps a little but not much. The drought for coins and fish usually come in waves, it’s whenever you have just trained a dragon to three stars, or perhaps just upgraded one of the facilities. It appears that if you don’t spend any runes on recharging energy, the drought will automatically go away, as when you speed up the game, you suddenly have a lot more uses for the resources a lot quicker.
When all of your roster have been maxed out in levels but you are far from having enough dragon scales or duplicate dragons to train them up, that’s when you’d have an abundant amount of fish and only in comparison, a drought in coins.
Another suggestion is to not engage in the fish hunting quests and only do the coin hunt, and invest the rest of your energy in the repeatable quest in the current stage of your game to maximize coin gain.
I sometimes cheat the system when I have a full egg stock. I would put as many eggs in the hatching queue as I can, and then go to the repeatable quests to get more eggs, then remove all eggs from the queue, this way I can keep a total egg stock beyond the specified max. This might be helpful when you want to save up some coins but still want to get more eggs from quests.