Fungal infection in-between toes usually happen if the feet are in a moist environment for too long, like sweating, water and the such. Please avoid wearing closed foot wear when possible. You may apply an anti-fungal cream like terbinafine , in-between all the toes, both feet twice a day, for at-least 3 to 4 months, or till complete eradication of the infection.It would be good if you could consult a Dermatologist, who can send the fungus for culture and sensitivity test, and advise the topical and if required, oral medication as the test report.