Inexhaustible Vein Achievement
Earn 1,000,000 coins
This is a cumulative lifetime total earned. The only thing that does not count towards this total is if you sell fish or decorations to get some money back. This total only comes from money earned from completing levels.
This may seem like a massive amount when you start out, since you'll only be getting a couple hundred coins per level, but eventually you'll be getting multiple thousand coins per level. The ways you can influence your money earned are by completing a level within the golden time limit, and by leveling up. You get a gold time bonus and a level bonus when you finish a level, and these can be significant. This is why it's important to not hoard money. Spend it to continually level up, so you continually make more money. You should also be shooting for gold times on every level, although the difference between a gold time and silver time is pretty small, so don't worry too much about that and definitely don't replay any levels to try for gold times.
This achievement will come naturally as you progress to level 48 for your last achievements. You'll likely unlock this somewhere around level 40 as long as you keep leveling up whenever you have enough money and continually get gold and silver times.