I agree with the others about more and larger water changes.
On small tanks like yours more, is always better, and very easy.
I normally do a 30% to 40% water changes every other day, on hundreds of gallons of tanks.
Also, never throw away biomedia, swish it around in old water to remove grunge and put it back.
The most important component of filtration for the health of your fish is from the population of beneficial bacteria that live on biomedia, without it the tank water becomes toxic with ammonia.
The mechanical filtration is basically for "your" atheistic enjoyment, but hardly important for fish health.