Thermometers. Those liquid crystal strips you stick to the outside of the glass prove to be pretty dependable, though they don't last forever. "LCD", for "liquid crystal display", won't be confused with a "digital" thermometer by anyone under forty, I should think. When shopping for them, it's always reassuring to see that all the thermometers at the LFS agree, within two degrees. But if they don't, look them over and buy one that gives an average reading. Or choose another type of thermometer: a single expensive high precision aquarium thermometer from Tropic Marin (for example) could check and calibrate all your cheapies, and those of all your friends: stick a plastic label printed with the plus-or-minus conversion factor right next to each external thermometer strip, if actual temperatures are critical.
The LCD strip needs to be above the level of the gravel for an accurate reading, but it doesn't really have to be smack on the front pane. I've never found that the room temperature affected the reading I get from these strip thermometers, a caution I sometimes hear, but then I don't maintain a discus tank in an unheated woodshed, either. Nor do I have acrylic tanks: acrylic is a better heat insulator than glass, so these stick-on thermometers may prove to be marginally less accurate for you, if your tanks are acrylic.
Direct sunlight throws off any thermometer. Longterm exposure to sunlight can burn out these liquid crystal items, I've found. Tetra makes a stick-on liquid crystal thermometer where the word "Tetra" is larger than the numbers of the temperature scale. Damn! It's like your aquarium is signed Tetra. I take scissors and snip the name clean off before sticking it on my tank; nothing personal: I've never worn a t-shirt signed Prada either.
You do also need one movable floating thermometer, but keep it away from your quarantine tank, of course. Mercury thermometers are recognizable by their silver. Red thermometers use colored alcohol. You wouldn't want to break either kind in the tank.
With digital thermometers, better read the fine print. Cheap versions on the market may offer a register calibrated in tenths or hundredths of a degree, but may only be accurate within a degree or two. Jim Schmidt posted at AC, Aug 2001: "For $10-20, you can buy a digital, indoor/outdoor thermometer that works well in the aquarium. The best ones I've found are from RadioShack. The temperature probe for the 'outdoor temp' is sealed against water. You simply insert that probe into the water (I attach mine to a filter intake tube to hide it) and put the readout somewhere convenient. Every digital thermometer I've used has been accurate and reliable (compared to several alcohol-filled thermometers) and they have large digital displays, which makes keeping an eye on temps much easier. The also have min/max recording features, which allow you to see the high and low temps registered; this is handy to make sure tank temps don't drop too low or get too high (e.g., if your home heat goes down at night during winter months, you can make sure your heaters have enough oomph to keep tank temps stable). I've got about 7 of these thermometers in use for about a year and have had no failures yet."
Temperature settings. Fifty years ago, 74oF was an average temperature for tropical fishes. That temp figured in calculations for space requirements for aquarium fishes. "'Tropical' gives the false impression that all these fishes come from torrid climates, and that they must be kept at steaming temperatures," wrote William T. Innes in 1935, explaining his book title, Exotic [rather than Tropical] Aquarium Fishes. Average temperatures have risen over the years, like concert pitch: I'd say now that an average temperature is often considered to be 77oF.
Of the fishes I've kept, only gouramies and rams and apistogramma really need the warmth to flourish. My aquaria have heaters in winter set at 66o as a fail-safe for those inevitable days in September and May when the landlord isn't legally required to send up steam heat, though the day is distinctly nippy. A cool winter, hovering around 70-72o, and a warm, sometimes steaming hot summer have certainly suited my two Botia (now Yasuhikotakia) modesta, who celebrated fourteen years with me in 2012.