Giving the medicine to the bird by mouth, individually is the best way to be sure the bird has got proper dosage. If you wanna give them in drinking water then get liquid enro. Enro tabs don't dissolve well,hence settle down at the bottom. have you got Merriquin(brand)??
If the bird is 300 grams, then 6 mg would be fine, if 400 grams, then 8 mg. But without knowing the exact weight, can't give you exact amount. So 7 grams would be in the middle for a regular size pigeon, if you cannot weigh them.