WHIRLWIND EXPLANATION: Pokémon have a fixed, permanent score from 0 to 31 for each stat that affects how good that stat can ever get.

This calculator will figure out that score (called a "gene" or, more obtusely, an "IV") for you, so you can discard the unworthy. It's more accurate for higher-level Pokémon, so it's helpful to go into a level 100 wifi battle with someone and check your Pokémon's stats from there.

Stat calculator

Staryu
Staryu 		Base
stats		 Stats Effort Stats Effort Possible genes (IVs)
Level Level
HP 30
38–44
18–23
Attack 45
21–26
10–15
Defense 55
24–30
2–6
Special Attack 70
30–35
22–26
Special Defense 55
24–30
18–23
Speed 85
35–41
8–13

Hmm, I need more information to figure out your Pokémon's genes exactly. Try raising it to level 19 and entering its new stats.
Remember: if your Pokémon battles at all, you'll need to track the effort it gains. Consider saving your game and using Rare Candy instead.

And for your copy/pasting pleasure:

18–23 HP; 10–15/2–6 Physical; 22–26/18–23 Special; 8–13 Speed

or start over