Good IV magikarp to evolve?
I'm super new to checking IVs (only level 28 and previously just wanted to complete my dex) so I'm not sure which magikarp if any to evolve now:
83% level 15, 82% level 31, or neither- should I wait til I find a better one because it takes so much candy? I have one shiny gyarados but it's got garbage ivs so I'm hoping for a good one next.
Answers
I don't know how familiar you are with how IV's work, but it is a widely misunderstood subject, and they matter less than most people think.
Gyarados's base stats are 237/197/190 (A/D/S). The IV's get added to those numbers, so an 80% Gyarados with 12/12/12 (80%) IV's has adjusted stats of 249/209/202. A 100% perfect Gyarados has adjusted stats of 252/212/205. That means that the 80% Gyarados is 98.7% as good as the 100% Gyarados.
People get way too hung up on IV's. My advice is to evolve the level 31 one, and TM it to the moveset you want, and have fun using it. And while you're at it, power up that shiny, IV's be damned, and show it off in gyms every chance you get.
ScooterJameson already gave you good advice here. I'll try to be a bit more specific:
* If you need a powerful water attacker now, evolve the lvl 31 82%, it's already a good one and will serve you well
* If you don't need it now and intend to power your best one up to the maximum, then you could wait to find a better one.
If you don't intend to power it up to the maximum, a 12/12/12 will be just as good as a 100% with something between 3 and 6 power-ups (especially since waterfall is a powerful move, so breakpoints are many and close to one another, and also are less important than for weak attacks).
That said, unless you really need it now, I would wait a bit to know what the imminent combat rebalance will imply. Other pokémon might get better, you never know! If your shiny Gyarados is already at a decent level, it's battle ready, you probably don't need another one right now.
Having better IVs often just mean fewer power-ups to reach the same performance. The 82% IV, level 31 will already be quite useful without spending any stardust.
If you are going to power up research catches from level 15, especially more common research like Magikarp which can be found with the "catch 10 pokemon" research, then yeah you can be a bit more selective with the IVs.
Sorry to derail but just so you know there is a research task to evolve a magikarp (I think it's the celeby questline). So if by any chance you haven't done that yet you should definitely wait to reach that research step, regardless of your choice about which one to evolve. 400 candies are quite a lot to grind so I should warn you just in case you didnt know