As long as you have a decent aerating(spelling?) system you shouldnt have to worry about keeping them until it gets really hot. At that point you might want to look into a product like rejuvanade or another livewell additive. The best thing is just a little bit of ice, but you have to remember to freeze your own ice using natural water not tap water. You really dont have to worry about it though until the water gets up around 80 or 85, a good aerator will keep them fine until then. Just be careful on making long bumpy runs with livewells full of fish, that can be as hard on them as anything