How often do you perform a water change? Once every 2 weeks
What percentage of the water do you change when you perform a water change? 25%
^ This is why your fish is dying. He's darting around because his tank is full of toxins and it's very likely he has ammonia poisoning, from all the built up pee in his water.
Put him in a covered cup with a little old tank water. Change 100% of his water and rinse the tank out ... now-ish? And then refill it with warm water as close to his usual temp as possible.
If you have any rock, kosher or aquarium salt, add 1 teaspoon (pre-dissolve it, make sure it's all dissolved before he goes in it) per gallon. Do this just for this one water change, it'll help him with the ammonia poisoning. If you don't have this, don't worry. Just clean water will do.
Because he needs some clean water, fast
Once you've got the clean water in and conditioned it, float his little cup in the tank to help acclimate him (get him used to the temp) and add a little new water to the old every few mins.
Tomorrow, do another 100% change - no salt, this time.
Please, please change your water way more often? This will really help your fish remain healthy..
ETA: If you can, purchase a product called Stress Coat. This will help him recover. It helps to heal the slime coat, which is what keeps a fish healthy. Ammonia erodes this, and makes the fish more likely to get ill.