Fish Health Help Please. I'm Stuck

Rams are one of the nitrate sensitive species; nitrate must be kept low with these fish.

As for filter jugs - be they Brita or another brand - they all work on the same principle. They soften the water using ion exchange resins. This means they remove the minerals that make water hard (calcium and magnesium ions) and swap them for something else. That something else could be hydrogen ions which would dramatically lower the pH, or sodium ions or potassium ions. But adding the 'something else' to the water is not a good idea. In the case of sodium and potassium, soft water fish have not evolved to cope with these so adding them is bad for soft water fish. And with a pH as low as my tap water goes in my Brita jug is also not good.
My tap water has low KH at just 3 german deg - the Brita jug dropped that to zero. So not only did it acidify the water, it also removed all the buffer. Where the KH is higher than mine, the filters shouldn't drop it to zero but they will lower it.