I have one 5.5 gallon tank with 2 (divided) bettas, and 2 more bettas, each in 1/2 gallon bowls (temporarily----they will get their own tanks after we move later this month.)
I was uninformed, and put the two in the tank with a filter and heater and thought that was enough. I now realize I'm lucky they lived. Apparently the tank is now cycling. I have rocks and silk and plastic plants, plus the mesh divider. But I have not done a water change since they went in the tank about 4 months ago.
I have been doing 100% water changes for the fish in bowls every few days. They have been fine (except for the one I accidentally killed when I forgot (apparently) the water conditioner.)
I finally became enlightened this week, and did a partial water change in the tank yesterday. I'm seeing posts on here about people who do 50% and/or 100% water changes on their fish in tanks. I'd really like to do a 100% water change, because there is so much crud in there---vacuuming yesterday stirred it all up----it was disgusting. It has settled back down, of course, but now that I know about it I want it gone.
I now know I need to test the water but I haven't used my new kit yet because I want that water gone either way because of all the crud.
However, I saw somewhere that 100% water change is dangerous. Yet obviously people are doing it.
1) In what way is it dangerous and what can I do to make it safe?
2) I read to put clean tap water in a bucket to refill the tank, and adjust the temperature with the faucets. I was taught by my mother (who was taught by her mother) to never use hot water from the tap for drinking because the hot water heater has corroded filth in it. I realize they may have been completely wrong, but I need reassurance.
3) Is there anything else I need to know?
Ann, my daughter Sarita, and our lovely fish: Ray, Aretha, Stevie, and Whitney