I wouldn't point a finger at current. Gars can live in moving water, but they also inhabit lakes and small ponds.
Nitrates are probably the #1 red herring for fishkeeping issues so it may not be as big a deal as you think (although yours do sound like they're way too high). That being said, compound stress (i.e. multiple stressors) often plays a role in fish problems so by improving water quality you may see improvement regardless of whether or not that is the biggest problem. And you did mention that the NO3 test was "very red", which on the API scale can be about 160+. If your tap water has 0 nitrate I would do another test with 50% tank water and 50% tap. Then multiply by 2 to get your NO3 concentration. If it still shows up red you could do a test sample that's diluted even more, but regardless of what that says your NO3 would be WAAAY too high and you're looking at multiple water changes and reducing your stocking density (e.g. sell fish or get a bigger tank).
Does the gar still eat? I see goldfish in the tank. Those are TERRIBLE for your fish because they are a huge contributor to fatty liver disease, which kills a lot of aquarium fish.
pH? Temp?
