I am writing this in desperation because I have had heater problems almost since the installation of my tank in August 2010. It's stressing out my fish and I've lost several to disease (first ich, now something I cannot identify).
Here's the situation: I have a moderately-heavily planted 45-gallon freshwater tank that is 24" high and powered by an HOB Marineland filter. It houses cardinals and corydoras. I installed a 300W Marineland Visi-Therm heater in August and monitored its function...I set it at one temperature and put a thermometer in one corner towards the top. The temperature of the water was consistently below the temp setting on the heater, even when the light was off. I contacted Marineland's customer service and they promptly replaced it.
I installed the second one. It seemed to better for some time. Then one nigh in the middle of the winter (house ambient temp around 56 degrees), the temperature dropped. I woke up and the tank water was the same as the ambient temp and the heater was off. I still had the old heater, so I stuck that one in hoping that 2 300W heaters might keep the temp stable. I also kept the ambient at 66 degrees even at night after this incident. This method wasn't terribly efficient - I set both heaters to 80 degrees and the water stayed around 78. So I raised it to 82 to get a water temp of 79-80...at least it was stable. (I lost 9 cardinals to ich a few weeks after the first heater "failure").
Then, recently I had another temp drop - it dropped to around 70 degrees. This happened during the day, and I found out in the afternoon when I was doing a water change. By this time I had bought another heater but I had not installed it because the other two seemed to be working ok, albeit not perfectly. This heater is a Hydro or something like that...can't recall the exact name, but it's also a 300W heater I believe. So, I replaced the original Visi-Therm with the brand-new Hydro. Even still, I'm seeing another outbreak of infection of some sort (not ich...not sure what it is) in 2-3 cardinals and a cory. My water levels are good - nitrates always under 10; usually under 5; ammonia and nitrite 0; pH stable at 6.6-6.8 and soft water.
Both heaters are placed horizontally in the tank, one is up a little higher and towards the left, the other a little lower and towards the right. I still have the same problem with inaccuracy - the thermometer combo seems to be precise (it keeps my water at 80), but not accurate (they are both set to 82).
So, I need some advice on a better heater setup for this tank. Why am I having all these problems? I know water circulation might be a problem - it's a tall tank with an HOB filter. But actually, it would be ideal if the upper half of the tank were hotter than the lower half (cardinals like it a little warmer than corys). The Visi-Therms have gotten great reviews on here so I can't believe it's heater malfunction. I want to buy an Eheim Jager heater (I had one of these 8 years ago and I never, ever had problems...it still works but I am hesitant to use it because it looks like it has a crack in the glass, probably happened in storage.
One of the issues I am trying to consider is the proxiimity of the heater to plants - they can't move themselves and I don't want to burn them - thus the reason for keeping the heaters horizontal and up high (even though they are visibly not so pretty).
So...advice? What changes can I make to heat my tank accurately and precisely? This temp fluctuation is stressing my fish.
Thanks in advance for your help and advice, as always.