The final decision has to be yours but it does sound like she's suffering or at least at a point where recovery is unlikely. Nailing the cause down to the phosphorus is next to impossible. It does sound like she has been on a more fish diet then anything else. That maybe more of an issue in my opinion but this isn't the time for that.
Rita needs some sort of help whether it be a Vet. (I don't think there is much they can do at this point) or putting her down.
Garters are very good at hiding(not showing) health issues until it becomes critical. With that in mind a keeper must realize there will be unexpected deaths that will never be explained.
Prayers are going out to both you and Rita.