Hello there. I have had my betta for about 6 months in a 1 gallon bowl. I heard that bettas are supposed to be pretty active but mine mostly just lays on his plant. He doesn’t greet me with a wiggle like other people say their bettas do. I tested for ammonia like you said and it was zero. I change his water every 4 days with bottled distilled water. I also noticed his color isn’t as bright as it used to be.
A: The cause of betta lethargy can be difficult to pinpoint because there are so many environmental factors that affect them. What stood out to me immediately is that you are keeping him in pure distilled water. Unlike tap water or spring water, distilled water has gone through a process that strips it almost completely of all minerals. Like RO water (reverse osmosis), it is essentially pure and no longer contains any of the trace elements that would be found naturally in water. Fish kept in distilled water over a period of time may show signs of lethargy or color loss like you described. Most aquarists choose to use tap water because it is very inexpensive and if treated with a good aquarium conditioner is safer and healthier for your fish then distilled. If you have tested your tap water and found that it isn’t suitable for fish (extremely high or low pH for example) you could buy bottled spring water. Spring water has not gone through the distilling process and still contains the minerals bettas need. It is considerably more expensive then tap water but usually less expensive then the distilled water you are currently buying.
Temperature fluctuations may have also contributed to your bettas lack of energy. An small unheated bowl can fluctuate several degrees from day to night. I find that a 2.5 gallon tank with a 7.5 watt heater or a 5 gallon with a 25 watt heater will hold the tank stable so the temperature isn’t able to drop at night. Because they are cold blooded animals, bettas can become lethargic very quickly when temperatures fall. When the temperatures fluctuate several degrees over a short time bettas can become stressed, slow down, loose color and even become quite sick.