How do I know if my fish are stressed or unhealthy?

A stressed or unhealthy fish usually "tells on itself" with changes in behavior first. Watch for fish that hide all day, clamp their fins tight to the body, gasp at the surface, hang out right by the filter return, dart around like they're spooked, scrape against rocks or wood (flashing), or stop coming up for food. Another big red flag is a fish that's suddenly isolating from the group or getting bullied when it didn't before-healthy fish act confident in their normal routine. Next, do a quick visual check for faded or dark color, frayed fins, white spots, fuzzy patches, red streaks, swollen bellies, pinecone scales, cloudy eyes, or rapid breathing. Also pay attention to poop: long clear/white stringy poop can point to internal issues or not eating enough, while normal poop is usually darker and drops off quickly. If you see any of the above, your first move is almost always to check the water. Test ammonia and nitrite (both should be 0 ppm), nitrate (ideally under 20-40 ppm), and confirm temperature is stable for the species (most tropical community fish do well around 24-26°C / 75-79°F). If ammonia/nitrite show up or the tank smells "off," do a 30-50% water change right away, match temperature, and add extra aeration. Many "mystery illnesses" are actually water quality or sudden changes. A good rule: if a fish is acting weird for more than 24-48 hours, not eating for 2-3 days (less for small fish), breathing hard, or showing visible spots/sores, treat it like a real problem-test water, correct it, and consider a simple quarantine tank if you can. If you tell me your tank size, fish list, temp, and your latest test numbers, I can help narrow down what's stressing them.