Any variety of white fish can be substituted for the red snapper. Some people like to substitute fish head for the fish fillets. If using fish head, have the fishmonger remove the gills and scales.
Substitute kangkong or swamp cabbage for the pungent mustard greens if desired. You can omit the white radish. You can add more vegetables like yard-long beans.
Water that has been used to wash rice can be substituted for the plain water if desired.
Sinigang sa kalamansi powder or sinigang sa sampaloc (tamarind seasoning mix) can be substituted for the Filipino lemon juice.