Research has shown we have 3 degrees celsius to play with. For the vast majority of tropical fish, a 3 degree drop is acceptable, but beyond that, it's a health danger. For most species, if it goes up, that isn't a short term issue, but down is dangerous.
If it falls very gradually, that's a different story. Then, your limit is the tolerances your fish are evolved to live within.
So as I pour from the hose, I keep touching the water flow to gauge where we are. Canadian winters have taught me that. I tend to like to keep my fish on the lower side of their natural water temperature range, so I don't give myself a cushion there. I use a potable water quality hose attached to a faucet, and run a mix of hot and cold water into tanks I've just dropped measured dechlorinator into.
Since I have a full sized (maybe oversized) fishroom, I have it divided into sections. In winter, I have 4 blocks that take about 30 minutes each to water change, and I do one a day early in the morning. That way, when the water coming in from the lines is ice cold, I never run out of hot water from the small water tank. In summer, I can do the whole lot at once, although I tend to divide it up because 2 hours is a solid block of time to have free.
Where you live matters. I get very clean water, which is wonderful. I don't have chloramines, just old fashioned chlorine. I get no nitrates and no measurable water pollution from the tap. My energy costs are climbing, but okay. But I do get cold water out of the tap for 2 or 3 months, and cool for 3 or 4. I need to mix hot in all year, but how much varies. In January, a lot.
If it falls very gradually, that's a different story. Then, your limit is the tolerances your fish are evolved to live within.
So as I pour from the hose, I keep touching the water flow to gauge where we are. Canadian winters have taught me that. I tend to like to keep my fish on the lower side of their natural water temperature range, so I don't give myself a cushion there. I use a potable water quality hose attached to a faucet, and run a mix of hot and cold water into tanks I've just dropped measured dechlorinator into.
Since I have a full sized (maybe oversized) fishroom, I have it divided into sections. In winter, I have 4 blocks that take about 30 minutes each to water change, and I do one a day early in the morning. That way, when the water coming in from the lines is ice cold, I never run out of hot water from the small water tank. In summer, I can do the whole lot at once, although I tend to divide it up because 2 hours is a solid block of time to have free.
Where you live matters. I get very clean water, which is wonderful. I don't have chloramines, just old fashioned chlorine. I get no nitrates and no measurable water pollution from the tap. My energy costs are climbing, but okay. But I do get cold water out of the tap for 2 or 3 months, and cool for 3 or 4. I need to mix hot in all year, but how much varies. In January, a lot.