For money making heres the easiest thing to do
1 Buy as much ornaments as possible
2 put a fish that you want to sell in the tank that shows you their age.
3 Click species
4 Make the price The highest you can make it
5 Repeat steps 234 on the rest of the fish you want to sell
6 Click sell fish and they will sell like hotcakes
That concludes my cheapskateness