I would start with 1 tsp per gallon, fill a small to medium glass with tank water, put the epsom salt in that. Once completely dissolved add about 1/2 to the tank & then add the other half about an hour or two later. I don't know your water change schedule but you ONLY replace the salt for amount of the water changed; example you have a 2g tank & do a 50% (1g) water change you add back 1 tsp. Typically it only takes a few days for the epsom salt to work but could take longer, if the 1 tsp per gallon isn't working after a couple of days then you may need to up it to 2 tsp per gallon. The behavior you're describing doesn't sound like she's weak but sounds like SBD. If it makes you feel better give her one or two pellets then fast & add epsom salt. If you don't see any improvement or she gets worse let us know immediately.