This is by my experience but I didnt do it alone. Got lots of info from others here and sorry if there is some mistakes in there. Plz help.....

STEP 1
Choose an 100 empty aquarium and choose a type of fish. You need to do this to all of them. Make sure "fewer clicks" is ON in options. You are gonna do the buy / resell method. So buy 25 packs of cards. Click on it and let them open. LISTEN to the sound! You will hear cards sound and some bells. A higher bell sound means you got something you dont have.

IF YOU HEAR THIS SOUND
The sooner you hear the sound it means the fish its close to the borders. Click in the middle and move the mouse right and left so you absorb the cards. Look for this fish (or several). It has a black contour although rainbow doenst seems so. If you find it, take a note. You can also look for rarity and color in your fish panel. Get this fish and move it to another aquarium. Resell the rest. Buy another package and repeat. The fish you found you need to grow to adults. You might wait to have a few but the faster you grow them, faster they go to your collection and will not show again to you as much.......

IF YOU DONT HEAR THE SOUND
Move your mouse fast left to right (any tips here?) and sell all and buy another package again. Keep repeating. More you do, more you get. There is a progression so you get more chance for every package you open.

DOES IT GET EASIER?
Seems so. More fish you have in common or painted, more gold you have. Regarding rainbowns I dont know. but it seems so too......

KEEP A POSITIVE BALANCE
Spring pals and giants are expensive. But by now you should have several aquariums. Use them to grow those and resell. Then you can keep a positive balance.

I HEARD THE SOUND BUT COULDNT FIND THE FISH
In this case no worries. You have several aquariums right? Grow this one to adults. When thy get to adulds you will know. And for the time beaing work on other aquariums!

Tks all and sry if I got any info wrong. Just started the game but I am sure its working for me :)