Hi, I am relatively new to the game, I have a couple tips to share for keeping track of some rather pesky details.
For fish that you want to sell, price them on a number ending in five.
For your fundraising fish (cash cows) you can end the price on zero.
For fish that you want to save, add 9's to the back of your modified price, until all four digits are used. Several reasons for this: first, it will overprice the fish and it won't get away from you in the sales tank. Second, it is another way to tell which fish are most valuable to you for whatever reason, regardless of their resale value. Third, you still have a vague idea of the original price of the fish, just take off the 9's if you need to sell the fish and it will still be a reasonable price.
Hypothetical example, early in my game I was selling greenfin betas for awhile, a whole tank bred to sell. If their original price was 27 dollars (I remember it was close to this amount), I would mark it up to 30 if I wanted to breed them consistently to raise money, 25 if I wanted to sell them sporadically, and 2599 or 3099 if I had ONE that I wanted to hold onto.
The point being to create a code of sorts that stands apart from the suggested prices, and gives you information about the value YOU place on your fish for your own purposes.
Don't forget to price your fish up as your tank improves, better advertising and more doodahs in your tank will increase the chance of selling at a higher price.
I also have a sorting trick for use when moving fish between tanks and trying to get a sales tank full. This is when you are trying to sell off duplicates without selling the last one of anything and having to recreate it. Shift fish around between tanks ONE and TWO until fish families are all together, such as all the comets together, all the bananafish, etc...may have to work one family at a time and sell some to get wiggle room then repeat. Then focus on one family and, one species at a time, put ONE in the breeding tank (tiny tank at the bottom) and remove all the other identical fish to the sales tank. Then move the fish in the breeding tank back to tank ONE or TWO, whichever makes sense in your other pursuits.