Should an Aquarium Heater Touch Gravel?

By Nadine Oraby | 2020 Update

Aquariums are amazing little worlds, and part of the enjoyment is trying new things. The right things, we hope. Heaters are needed for some popular fish in the hobby–they are called ‘tropical’ fish, after all—but electrical heaters are not toys, and it’s important to learn their limitations.

Should an aquarium heater touch gravel?

The answer about whether it’s a good idea to let your heater touch the gravel substrate is “no.” This is the rule for any glass-encased heater, though there are related issues and not every heater is the same.

The most common aquarium heater is sealed within a glass tube that hangs upright, bolted with plastic screws to the side of your tank. There are submersible heaters too, for a slightly higher cost. In either model, the heating element is contained within the glass to insulate your water from the unit’s live circuit.

It is imperative this glass tube is not cracked or broken, because even the slight depth of 6″ will force water into the housing–which eventually comes into contact with the heating coils. If you look at a heater, you’ll notice it comes with a specially built protective cage to prevent the heating element from touching even the glass side of your tank. Take heed of this.

You may not notice cracks if they are tiny, and your fish will not encounter the live current (as they are encased in non-conducting aquarium glass) … but when you go to feed or otherwise touch the water, you are subject to the shock.

Even if the heater is placed gently against the gravel substrate, it is still in danger. This is because the heat differential between the untouched portion of the heater’s glass tube and the area in contact with the substrate can stress the surface and cause micro-cracks. For this reason, make sure the heater’s surface is free and clear of all obstructions.

YOUR Freshwater Fish Tank Probably Doesn’t Need a Heater! (Video)


Heaters Explained

Even if you’re new to the hobby, you’ve probably heard the horrible and sadly often true stories of mishaps with a broken heater. You should respect this device and learn how to handle it.

There isn’t much in your innocent aquarium that is able to hurt you, but an exposed live electrical heater may be one. To stay safe, there are a few precautions you should take with any heater or other electrical appliances in the aquarium.

1. Respect the Heater

Electricity and water are not an easy combination. The conductivity and corrosiveness of water are so dangerous in conjunction with an electrical current that it’s difficult to manufacture a completely safe device.

Even so, the average aquarium heater does a pretty good job, if it is cared for properly. The glass housing seals the electrical parts from water, and as long as that seal holds the unit won’t electrocute you and your water friends. One tiny crack that goes unseen can open up a Pandora’s Box of grief, a process that begins with wasted money and can end up with dead fish … or worse.

Obstructions touching the heater’s glass are trouble because surfaces conduct heat differently. Any material that touches the heater can create cooler/warmer spots on the tube’s surface, leading to a variance of expansion on the glass. Over time this can produce minute stress fractures that let in water and disaster.

Even letting the heater touch the hardscape and plants is a bad idea. Nor do you want to dig a hole in the gravel to make room, as the substrate will fall back in overtime.

If you have a tank in which the heater doesn’t fit without coming into contact with your gravel or décor, get a new heater. Or a new tank. It’s not worth the risk. By the way, you can’t solve the problem of an overly long tube by exposing part of it: conductivity of air will cause the same issue. If you do mistakenly re-submerge a heated glass tube, take a picture of the webbing of fine cracks it creates to post online because that’s all the heater will be good for.

2. Another View on Heating the Gravel

Placing your heater against the gravel is bad for the reasons explained, but there’s actually precedent in the hobby for heating the substrate. In fact, not many years ago the concept of gravel warming was a serious practice.

In his book Creating a Natural Aquarium (2000), Peter Hiscock outlines the theory behind the concept. Special heating cables are buried under the gravel or sand bottom, with the idea that thermal currents will form in the substrate and, as the warm water rises, circulate in new oxygen and nutrients. Proponents claim this keeps the gravel healthier, and prevents dead spots caused by anaerobic decay.

The practice has waned because there hasn’t been proof of this actually working. There are aquarium keepers with booming planted tanks who credit heated substrate for their success, but others who have tried cannot duplicate their success.

Likely you are not attempting to revive this practice, but if you do decide to experiment with heating your gravel, obviously don’t do it with a glass-tubed heater. Invest in special cables instead. And keep your regular heater too, because if history is a guide you’ll be switching back soon.

3. Sabotage in the Tank

We tend to think of our tanks as static systems that only we can change, and for peaceful community tanks, this may be true. But inhabitants sometimes have their say in surprising ways.

It’s important to periodically check your heater to make sure it hasn’t slipped or had a bit of hardscape topple over on it. Some fish dig and can disturb the substrate or make unfortunate mounds that partially bury the tube.

Be especially vigilant if you keep larger fish. Cichlids, in particular, have been known to take affront at the presence of a heater (or anything) in their quarters. They can attack the offending device with foreseeably poor results.

The lesson is to know your fish and adjust accordingly. House the heater in a suitable protective cage if you have destructive inhabitants, or place it in an outside filter or sump–or get an inline heater that isn’t exposed. Of course, you’ve been reading this article, so you know to never stick your heater into the gravel to shore up its position.

4. Safety Precautions

There are simple but crucial precautions to take with all the electrical appliances in your tank. Powerheads, skimmers and the like can cause problems, but the most critical is the heater since it is the easiest equipment to break that brings high voltage into your aquarium.

  • The first precaution is to install a ground fault circuit interrupter (GFCI). This inexpensive item can be installed in your wall outlet, or be built into the cord you use to power the heater.
  • A grounding probe is another cheap and simple safety device to install. The probe goes into the aquarium, and the other end plugs into a grounded outlet. This provides a better conductive path for the current than your body. It also keeps your fish safe from minor current you don’t detect.

It’s important to do both because these devices work differently and provide more comprehensive protection. They are cheap, and you’re worth it.

5. No Soup for You

We should mention the main complication with heaters: most will fail at some point, and when they do, they often stick and keep heating the water until your beloved aquarium is bouillabaisse.

Heaters disprove the free-market theory. For decades the industry has been swamped with cheap heaters, because mass production doesn’t favor quality. This tragic flaw has left countless aquarium owners broken-hearted, and many have left the hobby. People want to enjoy a joyful ecology under glass, not watch it become a death trap.

You can buy an expensive thermostat to monitor the water temperature. Another common preventative is to buy 2 heaters rated below your tank’s size so that together they keep your tank heated properly. When one of the heaters sticks, the theory goes, the other will itself shut off, allowing more time for the aquarist to discover commerce’s latest trick.

Question: I want to use the heater I have … but it doesn’t quite fit. Is there a safe solution?

Your options are limited with an upright model that bolts to the rim, but a submersible heater can be angled and/or placed at different levels in the water. The same “no obstruction” rule applies, but a submersible unit has more flexibility of placement.

What is the safest heater?

Big names in aquarium heaters are Fluval, Eheim, and Aqueon. Be aware there are no completely fail-proof heaters, though there are quality manufacturers. It’s your decision whether it is more economical to buy high-end heaters and replace them periodically, or to rely on a thermostat system with cheap-o models that burn out quickly.

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