Reddit mentions: The best liquid level sensors

Basically, I bought a float switch like this http://www.amazon.com/Aquarium-Water-Control-Plastic-Switch/dp/B005FDT02Q/ref=sr_1_6?ie=UTF8&qid=1406740109&sr=8-6&keywords=float+switch and clipped it to the rim of my tank so that it activates when the water gets higher than it should under normal operation.

There are a lot of different ways to wire it up so it shuts off your pump. Perhaps the simplest would be to connect the float switch to a relay that controls pump power. In my case, I have a custom built reef controller that I made using a Raspberry Pi. It does a whole bunch of stuff, including monitoring levels, temps, Ph and texting me when issues arise. It was relatively easy to wire the switch to the Pi and modify my program to shut off the pumps if overflow was detected. It also says "Main tank overflow detected, shutting down pumps" in a cool robot voice ;)

http://www.vellemanusa.com/products/view/?country=us&lang=enu&id=350507

I use this with my dehumidifier. in summer. I use automotive fuel pump form junkyard to pump water out.

when water level hit high mark pump activate. When it get low pump deactive.

Use bare wires instead of of PCB.

Easy to put together and works great!!

http://www.amazon.com/Velleman-K2639-Liquid-Level-Controller/dp/B0086GAGEU

I'm interested in the same thing. I've only gone as far as to install one of these to get alerts on water level:

http://www.amazon.com/Aquarium-Water-Control-Plastic-Switch/dp/B005FDT02Q

I have a sump though so it's probably much easier to put one down there than on the top tank.

You want something like this.

https://www.amazon.com/dp/B00FHAEBIA/ref=cm_sw_r_cp_apip_Sr8bw6syd6DDq

I use it to make sure the reservoir for my plant watering device is full.

This is my vote for sure. You can monitor the water level in the sump pit easily with a float switch. Something like this works nicely:
https://www.amazon.com/dp/B01LYCTVT1

I'm putting a pair of these in a cistern to control turning on/off the well pump. I'm also adding a second pair low and high water alarms.

Standard float sensor on a gpio of an esp8266, could maybe even hack it into your existing smart plug

Edit

https://adosia.io/product/water-level-sensor-switch-horizontal/

Gikfun M8 32mm Liquid Level Sensor Fish Tank Water Float Switch PP Plastic Ball Float Controller DIY Kit for Arduino EK1373x3 https://www.amazon.com/dp/B07G13JB57/ref=cm_sw_r_cp_apa_i_WNrnDbVD86S56

Could you get some heating blankets and put them on the shelves? Use some PID temp controllers + scheduler/timer? You could even use something like the Wemo Insight Wifi switch to understand the usage/cost and control the system. They actually make these which seem to be designed for plans

Another option would be enclosing the area with the orchids and using a small space heater to keep the area warm, you would probably still want a few fans to keep air moving, and you wouldn't want to seal the area air-tight and suffocate them.

Issues with your system:

• Make sure you use a temperature controller. You wouldn't want the water to get to hot (and shock the plants) or boil the water.
  
• You could use an off the shelf small hot water heater or instant water heater (maybe something you can get used or second hand).
  
• Something to disable the system if the water gets low (evaporation or leaks) so the heater doesn't overheat and melt or start a fire. Like a float switch.
  
• You need a pump or something to move the water.

Termineter communicates with Smart Meters via a connection using an ANSI type-2 optical probe with a serial interface.

Price, $455

More info

I get ya... just one more idea.

You could wire a 24v transformer to a float-switch - then from the float to a 24v solenoid valve. When the float is triggered it will shut off the valve.

Maybe I didn't describe that well. Someone flushes the toilet, and the water in the tank drops. The water sensor opens. When the tanks fills up again, the sensor closes (detects the water) and then fires another relay one time to inject the solution, not into the tank, but the bowl itself.

The next flush, the water drops, sensor opens.....

I'm assuming here that the sensor is in a closed state if it is in water. I suppose a floating bob switch might work too.

EDIT: Do you think these might work? I'm not sure how the trigger.

http://www.amazon.com/uxcell-Pieces-ZP4510-Vertical-Switches/dp/B00FHAEBIA/ref=zg_bs_306931011_1#Ask

Thanks. FigJam did all the work. I kinda tidied things up.

Amazon has a bunch of styles. Look for aquarium or transfer tank float switch. I used these but they tend to fail quickly or are DOA. Here is the straight version. You will need to fashion a mount or stand for them.

Others can post what they used.