Aquarium Water Getting Way Too Warm

[super_awesome]

So I've had my 25 gallon tank (platies and tetras) for around 8 or 9 years now. It's multigenerational and overall an apparently successful tank. This is the first time I've had a concern about the water temperature being too warm. I dunno why, but it's been around 80+ degrees for the past couple of weeks, since the last water change. It's usually been low 70s, an even during the summer it wasn't ever over 73 or so degrees. So I dunno whats going on now, and the fish are seeming a little lethargic.

The heater has always confused me, it's pretty uncomplicated, with a little red tab on the top than you can try and turn to the left towards a + sign to increase the temperature. Honestly I've never touched the thing except on a few occasions (moving, full tank change, etc). There's no obvious option to decrease the temperature, and I'm worried if I take the heater out altogether, the temperature will decrease drastically to winter levels, shocking and killing the fish.

Any advice? I have no idea of the heater's brand or anything. Usually when I dip a finger in the water, it's a little cold, room-temperaturish. This is the first time I put my finger in and it feels warm. Is mid-80s too hot for the fish (platies, incl a little month-old guy and a ready-to-burst pregnant mama; cardinal tetras, an 8 year old cory and a couple botias).

I've also noticed that, since this heat increase, the amount of algae has increased a ton (live plants). Not sure if this is a cause, a result or non-related (I recently added a couple botias who have feasted on those unavoidable snails I've gotten from adding new plants).

I know this is a pretty dumb question but honestly I've never touched the heater. Thanks.

 

[Katty]

Is the heater 8-9 years old as well? Sounds like it's malfunctioning and needs to be replaced. Mid 80s can be tolerable during short term, but if the fish are acting different (lethargic as you say) then it's probably not good and you should try to fix the problem. Higher temperatures also means lower oxygen levels in the water, so if you can, try increasing the water circulation (turn up the filter, lower the water level to get some splash from the filter, turn on the bubbles, etc) to help with oxygen until you can get the heating problem fixed.

Since this heater can not be turned down, you can either try unplugging it and plugging it back in to regulate the temperature yourself, which is only a short term fix since most people don't live next to their tank, or just buy a new one.

What's your room temperature? Just wondering because our house is usually right around 70F, seems silly to me to have a heater to heat tank water to 73 F, but you probably live in a cold place then...

 

[WILDER]

I would invest in a new heater. As you could end up boiling your fish alive.
If it's a juwel heater they are know for over heating the tank as the heater gets old.

Till you get a new heater increase aeration.