Maracyn is erythromycin, which is typically dosed at 200mg/10 gallons. That said, you want 100mg for 5 gallons. What I do for my tanks is to dissolve the entire packet into a known quantity of water and then dose from that. For example, I'll dissolve the 200mg packet into 10mL of water and then draw up 5mL of the solution using a syringe and add that to the tank (you can dissolve it in any amount of water if your math is correct - a packet in 20mL of water can be easier to dissolve and you would dose 10mL of solution). Afterwards, I discard the rest of the solution because the medication starts to lose its effectiveness over time while in solution.
Or if you happen to have a scale that is accurate to small measurements, you could just weigh out the 100mg.
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