What fertilizer do you recommend for strawberries in containers? My plants are 2 years old and produce only small, misshapen berries.
One-year-old strawberry plants are more productive than older ones. If yours are on their second year, it may be time to root some of the daughter plants and replace the old parent plants. Plant them in good potting soil that contains slow-release fertilizer; you won't need to feed them until autumn.
In September, clip off all old leaves that have red or brown spots, and give the plants some water-soluble plant food. Allow the plants to go dormant through the winter by keeping the container outdoors in a protected spot, or in an unheated garage.
First thing in spring, move the container to a sunny spot and begin feeding the plants every 2 weeks with a general-purpose product. A few weeks later, you should have a beautiful crop of big, juicy strawberries.
From your description, I also suspect that your strawberries are being attacked by tarnished plant bugs. This insect feeds on developing blossoms. The toxin they inject while feeding causes misshapen fruits, sometimes called nubbins, to develop; the tip end of the berry becomes hard and seedy. Spray with an insecticide just before the blossoms open. Avoid using insecticide during bloom to prevent injury to pollinating insects.