As both linked articles (posts #3 and #4) mention, true planaria are usually harmless and controlled by regular maintenance, not chemical/medication treatments. Which brings up a point...are what you have true planaria, or one of the similar "false" planaria? That will make a difference re treatments.
If cyanobacteria is a problem, there are definitely very high organics in this tank, and that relates to the planaria (if true planaria). I still advise non-chemical treatments.
This isn't necessarily a general cure...it's for treating parasites. I would assume the metro is in there to treat secondary infections caused by the parasites as can happen? I didn't look at the safety data sheet though and I'm surprised to see that it said harmful to aquatic life even though on the package it says safe! Big sigh here......thanks for bringing that to my attention.
I realize this is for parasites, but it is still "general" in more than its name and these mixtures are sometimes not the best treatment. Metronidazole is an antibiotic that is extremely effective (on its own) in dealing with internal protozoan and parasites. It has also been shown effective with external parasites like ich, though there are some better direct ich treatments than metronidazole. [As it is an antibiotic, the too frequent use has to be curtailed as I mentioned previously.] I have more experience with this drug than most, as I have twice used it very effectively to rid a tank of internal protozoan, on the advice of a marine biologist and a microbiologist. It is best (much more effective) mixed in with food. But I cannot see any benefit against planaria.
One has to always know the product's side effects before using. When I have had un-diagnosable problems with fish suddenly dying, I have gone to a microbiologist online friend whose husband is a marine biologist (two birds with one stone, so to speak) and her first questions are always for the fish species, and then she recommends treatments that will be relatively safe. This is something most of us, including myself, are not likely to fathom without consideralbe training and experience. Which is why I so frequently advise against some "treatments." One can easily do more damage than benefit. All of these do stress fish, and significantly, not to mention worse side effects depending upon species and drug.
Do you think just good old aquarium salt might help get rid of these? I read such controversy on it and I would have no idea on what to start a dose on. But I know it would be safer on the tetras and the nerites as well. But not a clue if it would get rid of these. I MUST get rid of them ASAP before they explode more and start killing things. I'm so absolutely frustrated right now.
Salt and heat might work, I won't comment as I've no idea, but salt is one ingredient in that General Cure. One treatment for true planaria is to remove all fish, then raise the temperature to 35C/95F for several hours [this from
The Manual of Fish Health].