SeaChem Constant Alerts

To be honest, once the tank is cycled and mature, pH usually just stays the same. The exceptions are if you are using a lot of bogwood and have low KH; if you are using coral or limestone to increase hardness when you have low KH; you don't do nearly enough water changes and end up with old tank syndrome - especially if you have low KH.
You may notice a theme here    KH keeps the pH stable. You are only really in danger of  changing if you have low KH. Or if you add chemicals to alter it, which are not a good idea in the first place just because of problems with rebounding pH.

If you use RO water, you have to make sure you are mixing it with the right amount of tapwater or mineralisations salts. I don't use RO, but I'm sure that people who do check their hardness and pH more often than people who don't.

Seachem also make an ammonia alert, but I've read that it can take up to 4 hours to change colour  Someone else reckons it takes 15 mins to go up, the 4 hours is the time it takes to go back down after the ammonia had been removed by a water change. One advantage to the ammonia alert is that it only reads ammonia, not ammonia + ammonium like the liquid testers so it only shows the 'dangerous' amount of ammonia in the tank.
But unless you do something to kill your bacteria, or overload them in some way, ammonia should stay at zero. Are they useful for cycling? Not having used one, I can't really say.

Haven't come across these - they seem like a good idea though. Might have to keep an eye out for them