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Mind you this solution temperature we are talking about, not air temperature. Even if the water temps spike a tad you should still be okay, but once they go below 60 or over 80 degrees Fahrenheit you are looking at some potential problems.
Hot - Higher water temperature
High temps nurture harmful pathogens, fungus, bacteria and assorted microorganisms. Temperatures at or above 80F can easily lead to an explosion in their populations and an implosion for your plants. Dying roots caused by elevated water temperatures will exasperate the situation by providing a food source for many of the pathogens.
The dissolved oxygen content of the solution decreases as the liquids temperature increases, dissolved oxygen in the solution is essential for a healthy root zone.
Elevated water temps will also cause heat stress. The duration of the heat and the actual temperature of the water can easily place the plants in basically a survival mode. Root zones shut down and die back, plants wilt, fruit and flowers are aborted. Symptoms of rot caused by dying plant tissue sometimes appear as brown soft spots on foliage and surviving fruits. Some plants such as radish, lettuce, leafy vegetables and some herbs will bolt to seed.
Cold - cooler water temperatures
Cold nutrient temps, below 60 F are more common in outdoor hydroponic scenarios, indoor setups do sometimes experience it but it is more easily controlled. Cold temperatures will slow and stunt plant growth, under extreme conditions they die back or go into an artificial dormancy. The severity of the situation depends on the duration which they are exposed to low temperatures and how cold it actually is.
Water is slower to change temperature than air. The solution, if it started out warm will only gradually chill to meet the surrounding air temperature. In effect - so long as the solution is not exposed to cooler air temps for any length of time it should be within acceptable parameters.
Monitoring Nutrient Solution Temperature
Keeping track of the water temp in your reservoir is very simple, it only requires some diligence and a few minutes a day. A simple fish tank thermometer placed in the solution will suffice. Remember however that it's useless if you don't keep an eye on it. I like to write the temperature history in a log so I can keep track of my temperature fluctuations and compare it to plant performance . A fishing string is tied to the thermometer so you can pull it up when you are reading the temps.
Root Zone Temperature in some systems, such as flood and drain is a little more difficult to monitor. The roots are subject to the water temperature when flooding and the air temperature when drained so both should be in acceptable parameters and reasonably close to one another so as not to cause a shock for the plant.
Adjusting Nutrient Temperature
To bring down high temperatures you have to cool the solution gradually. A sudden temperature change can be harmful to some plants. Gradually meaning over several hours, not days.
1. Ice will suffice but keep in mind that you are also dilluting the solution as the ice melts.
2. The ice should be of the same quality of water as that used in the solution. Most 'city' water aka municipal water is treated, fluoridated, chlorinated and at times contaminated with substances, such as sodium and trace minerals that will not harm people very much when consumed as tap water, but over time can cause serious issues for plants grown hydroponically. See: Hydroponic Water Quality
A. Making ice from your solution itself is a nifty idea assuming you have the time to wait for it to freeze and that you are able to make enough ice to meet your needs.
B Another good idea that will eliminate dilution of the solution and water quality issues is by using zip lock bags or other suitable containers full of Ice to bring down temperatures.
If you intermittently have heat issues in your reservoirs the 'ice bottle technique' is advisable. By filling suitable containers with water and freezing it, high reservoir temps can be adjusted fairly quickly. Ice takes a long time to freeze, so having some ready to go is advisable.
Insulation can help keep heat in if your reservoir is loosing warmth to its surrounding environment. Keep in mind that if your system is getting too hot the insulation is actually working against you.
There are energy efficient nutrient chillers on the market that work well, depending on how much you want to invest and how extensive a hydro setup you plan on or already have you may want to consider investing in one.
Bringing temperatures up is not rocket science. If you have a cold solution you need to warm it up to an acceptable level. There are dozens of gadgets and home made gizmos made and concocted by wanna be Einsteins, some work, some ... not so well.
In most setups a simple fish tank heater will suffice, you do however need to monitor it as a high setting can quickly fry your plants. Heating the solution will naturally lead to accelerated evaporation so you'll also need to keep track of the water levels. A heater should not have to be run continuously, not indoors anyway, so consider putting it on a cycle with a timer.
I once used a heat mat designed for germination by placing it under the reservoir, it worked okay - just okay, not great. The drawback here is that it took a long time to bring up the temp.