I suspect that there is a build up of salts in the soil, likely caused by your water softener and compounded by fertilizer. Potassium, the last number on your fertilizer container, is one the the main components in fertilizer (nitrogen and phosphorous being the other two main components). Plants that are potted in a good quality potting soil rarely require fertilizing and only during active periods of growth. Manufacturers' instructions are for plants growing in optimum conditions, not homes, so when you do fertilize, best to dilute to half the recommended strength.

Water softener systems cause a build up of salts (minerals) over time which eventually can harm your plants. Some plants are more sensitive than others. My suggestion is to thoroughly flush the soil with distilled water until water runs clear through the drainage holes, then consider switching to distilled water for your watering regime.

Another thought is to check the discolored water and plants for unpleasant odor, which could be indicative of root rot, caused by overwatering or inadequate drainage. If you suspect root rot, gently pull the plants from their pots and carefully inspect the roots. Generally, healthy roots should appear white in color, firm and plump. Mushy, black roots are signs of root rot and should be trimmed before repotting the plant in the smallest pot into which the rootball will fit.

I hope I've given you somewhere to start. Someone else might have additional suggestions. Good luck and please keep us updated how your plants fare!

Peat-based soiless mixes commonly used for potting indoor plants contain no "dirt". If plants are potted in soil, the water will drain brown. Added coffee or tea is also a cause of brown water.