These pills are usually 400mg each. I read articles that suggested anything from 400mg/100L to 250mg/100L
I seem to remember using 4 pills in my 250L tank which was probably overkill but I didn’t know the dosage at that point and it certainly worked without harming the filters or fish.
Dissolve half the pills in about ½ cup of water. They dissolve very slowly as they have a coating on them. Give them a stir every now and then to speed up the process.
Once dissolved add it to your tank, swirling the water around to spread it quickly. The stupid fish will think it is food and try to eat it but don’t worry about that.
You should notice the cyano dying off within 24 hours or so. It is important to keep siphoning up the stuff as it dies or it will pollute the tank.
After 3 days, dissolve and add the other half of the pills, siphoning out the dead cyano remains often.
People worry about it killing the bacteria in the filters but I kept my filters running the whole time and noticed no difference. I was told the bacteria in the filters is the wrong sort and is unaffected by the erythromycin. Something to do with gram -ve and gram +ve bacteria.
Of course, the cyano may come back depending on why you got it in the first place. Mine was a result of adding infected plants so once I got rid of it, it didn’t return.
Good luck!