I have this paired with a Finnex 500w heater. This controller is easy to setup and has near perfect accuracy. I used a regular thermometer in the tank when I first purchased this to confirm accuracy. My only complaint is I wish there was a way for them to add a safety feature to know if the heater is out of water. I accidentally forgot to top off my sump and the water ran low which caused the heater to be exposed to air and almost start a fire in my house as it melted in the middle of the night. I realize there are ways of ensuring this doesn't happen and I have since made sure there is no way for this to happen again, but it would still be a nice safety feature to have.This unit sits outside the fish tank, allowing you to control the heating unit in the tank without getting your hands wet. It remembers your setting, so losing power isn't a big issue. The display is nice and bright and the numbers are big and easy to read across the room.To use it, you plug your desired heating unit (purchased separately) into the control box and drop the sensor from the control box into the tank at a place where it isn't near the heating element (you want the average temp in the tank, not the temp next to the heating element). Set your desired temperature and it will turn the heating element on and off to keep the proper temp.If you have big, nosey or toothy fish, you might need to protect the sensor in the tank, try hiding it with decor or putting a plastic tube over places they can easily reach. My two spot foxface duels with it a lot. My harlequin wrasse used to tug and chew on it and drag it around. Even though it has a sucker to secure it, they love playing with it.You may need to use a separate method to measure the temperature for a day or two as it can read a degree or two different than the water temp. I keep my tank at 79, but if I set 79, it goes to 80. Simple solution is to set it for 78 and it's always right at 79.The control box can control different elements. My box will run a 500 watt or 800 watt element. Bigger elements are needed for bigger tanks. Talk with a fish pro if you aren't sure what wattage to buy. I have an 800w for a 120 gallon salt tank.. It comes on and off a few times each day, for a just a little while at a time.Their heating elements are surrounded by a nice tough plastic so you won't burn the fish, or your plastic items.Would definitely recommend these over eheim in tank heaters for ease of use and accuracy.Easy to use and accurate from the factory - I currently have 2 Jager heaters connected to this controller. The temperature reading on this unit was fairly good compared to my calibrated digital thermometer. I've only used this for just over a month but setup was easy and the unit seems to be well built. First the unit itself is quite big, about the size of a small paperback book. The instructions for quick start and running was easy. The display shows the current running temperature and selecting set - will allow you to set the temperature you want to maintain. There is also a high temperature and low temperature alarm, which I have tested to be both working. First during leak tests and dumping cold water into my sump and then once when running hot water. My only complaint would be the suction cup used for the temp sensor - it does not seem to stick well and if there is any current around it, it could get dislodged easily. They do provide a stainless clip that helps secure the wire to the tank walls, but it also didn't really work that well. This is a minor flaw in an otherwise great package. Will update this review after some more time or if something happens to it.Been running this controller for a little over 6 months now with two finnex 300w non-thermostat heaters in my sump for a 125G freshwater tank. This unit has worked flawlessly so far. Love the high/low limit alarm. Comes in handy especially when dialing in your temps. The temp probe lead is a bit short however, the length should be as short as possible for accurate measurement. Also, tanks that require controllers usually run a sump system. My heaters are placed in the return pump chamber and the temp probe is in the chamber immediately upflow. That effectively puts the probe at the furthest point away from the heaters while physically only a few inches from the heaters in the downflow chamber. The lead length is perfect for this kind of setup. I monitor tank temp at either end of the tank, one in the temp probe chamber, and one in the sump chamber. I get no more than .5 degree difference between all the thermometers. This is one of the advantages of running sumps. Just ordered my second controller and a heavy duty splitter for dual 150 Aqueon heaters. Highly recommended and is purpose built for aquarium use in comparison to other controllers on the market that activate heaters and cooling systems.Have been running this on a 75 gal tank with a 300w Finnex heater for 5 years and 9 months. Controller finally failed (dead) without warning on Wednesday after powering off/on during tank maintenance/cleaning. Has otherwise been reliable and trouble-free. Opened it up - there is a fuse but it's soldered to the circuit board. Left without a working controller I ordered a new one instead of attempting a repair. Ordered Wednesday and had it up and running Friday night. Looking forward to getting 5+ years out of this one also, fingers crossed.This aquarium heater controller worked very well for 5 years. It stopped working suddenly and as it was well outside of its warranty I opened it up to see if it could be fixed. It had clearly had a small electrical fire or arc event inside the housing. With this in mind I am not sure if I would get another one.