If you want to progress here without spending a lot of money, level up your fish as quickly as you can. Rather than going by rarity classification (Legendary etc.) if you have a choice as to which fish to level up, choose the one(s) that will pay out the most first. You can tell which ones they are by looking at the cost to level them up relative to their level.
I currently have 221 total fish and only three that can be leveled up, which are my most expensive ones. Two are “Uncommon” and one is “Legendary”. They all cost 56,250 Fishin’ Chips to level up but the two Uncommon fish will upgrade to Level 3, while the Legendary fish upgrades to Level 5 for the same cost. That tells me that the Uncommon fish will have a higher payout level than the Legendary one, so I would level them up first. I recently did level up one of the Uncommon fish after completing MS123, which gives a gem reward. Now it will cost 75,000 Fishin’ Chips to upgrade that Uncommon fish to Level 4.
Fishin’ chips are used for leveling, but are difficult to get in large quantities. To get the biggest rewards from your Aquarium, most of the time you’ll need to level up your fish using a combination of Fishin’ Chips and gems. Investing gems in your aquarium will give you increased rewards from it, and bigger and better prizes in your bonus balloons. Help your friends increase their payout rewards by gifting them Fishin’ Chips 
Happy leveling!