I lost about 300 fish just a few days ago when my tank heater decided to stay on when it failed.(examination showed the theromstat contacts were fused together)So far I never worried about failing heaters as so far they just wouldn't heat anymore once the failed.This time I lost about $400 in living creatures and 2 years of breeding.Guess that is the punishment for being away for a few days - a bit over 50°C in your tank...Normal tank heater come in the submersible form or with the clear instruction NOT to submerge them abvoe a certain line - usually under the rubber capEither way they all love to fail within a few years if you are lucky, otherwise sooner.The one thing I always hated is that you need to find ways to hide them in your tank, in some cases you even need to a heat shield to protect sensitve critters in your tank.I though that there should be a cheap and easy way to eliminate most of the annoying factors....Simple, digital temperature controller....You find digital thermostats for your fridge and freezer for around $30-100 bucks, depending on the brand and features.Downside is they all activate the output once the temperature gets too hot.For under $10 you can get universal thermostat controllers- just a tiny white box with the display, two buttons and a few wires...Won't place a link but with the above you should find them on Ebay and Amazon for as said under $10.These can be set with the min temp being above the max temp - this effectively reverts the output state ;)Looks a bit weird when setting the temps this way but after that the power on the output is alive once the temp is BELOW of the set temp.And you can set the differential as low as 0.1°C if you like, but 2-3°C are fine for most tanks.Abandoning the internal heater thermostat....As the number one cause of failure apart from water getting inside we should not use the internal thermostat anymore now.To do this without messing around the temp is simply dialed way above what you will need for your tank.E.G.: If you want 24°C in your tank over the digital controller than the heater thermostat is cranked to 27° or higher.The on-off will be handled through the power our new digital controller provides.Sealing a heater....You should not have to do this but an awful lot of heaters simply fail because water makes it inside somehow.I use a bead of UV curing resin or UV curing epoxy glue for the first stage.The glass won't really expand, so the rubber from the cap won't expand much either.All we do is to add another layer of sealing protection.That stuff however is not always fully qualified for being suberged or just in contact with water a lot.So we add another bead of aquarium grade silicone on top.This also protects our tank from any harful chemicals the UV glue might otherwise release over time.Going external...With an external filter pump it just does not seem to make much sense to have a heater insde the tank.For some reason those filter manufactureres though about almost anything, including build in UV sterilisation - but not a heater...Using a bit of 5mm PVC drain pipe, two end caps and some hose fittings for your aquarium hoses we are set again.A bit of stainless steel wire from the gardeing section of hardware store helps too - the soft, flexible kind, not the really stiff and hard wire please!Design of the external heater:In one end cap you want to drill a hole of the same diameter as your heater tube, a mm bigger won't hurt too much but don't make so small that you need brute force to get the heater through the hole.Seal the inside and outside of the cap with silicone.If you want the heater to be replaced easily in case it does fail again (which it should not!), then you a pipe with a screw fitting and a screw on cap.Keep the assembly steady and secured while the (aquarium grade!) silicone cures - if in doubt let it cure for few more days.The lenght of the pipe should be so that there is enough space left in the bottom so the heater will have about 6cm until it would touch the other end cap.The pipe connections can be for one on the other end cap if you mount the pipe somehwere, otherwise both connectors should on the side of the pump and as close to the caps as feasable.Again, seal the inside and outside of these connections with silicone and let it cure.It helps if the fittings are of the screw in type with a backing nut.Downsides of having an external heater...You need to clean it out when you clean your filter, an added few minutes.If the heater thermostat is set too high and your pump is not running the water in the pipe and hoses will rise to the set temperature of the heater.For a medium sized tank that is no problem, for really small tanks it should be considered though - set the thermostat of the the heater not to the max if you think you might forget the water circulation one day ;)Upsides of having an external heater...The biggest benefit of having and external heater with a digital temperature controller is the lifespan.I used a dirst cheap 400W tube heater like this for over 6 years until finally the actually heating element failed.With the nicely illuminated temp display the times of checking the sticker on the tank or trying to read this tiny floating thermomoter are over as well.Possible upgrade options if you know how to solder or at least know how to correctly wire a relay...With an external tube comes the option to include a UV sterlisation lamp on the other end.The tube will be accordingly longer but a submersible lamp only costs a fraction of these ready made solutions that you still need to attach your your hoses somehow - and they are often quite big...Apart from that there is the option to modify a digital fridge thermostat (about $20-100 bucks) like the STL-1000.As most of them can't be re-programmed for other than cooling use and only have a single output for the compressor the internal relay needs to be replaced.Just select one that somehow fits inside once the original is removed and wire the NC and common output for your heater.Not in mood to fiddle with a controller and solder? No problem either!Just wire another relay to the output.Means the power that the controller switches on it will then switch your additional relay on.This gives you the option to use the proper contacts again to turn your heater on.Using a normal fridge controller this way however requires thinking the wrong way around for the temps.Remember!! : The frige controller "turns on" when the temp goes ABOVE a set temperature!!If you use it like this and connect your heater directly it means it will only turn off once the temp is BELOW the set temp and it will never again go below, only up and up and up....The added or exchanged relay however gives us the option to reverse this to some extent.You set the controller to the MAX temp that you want in your tank.Let's say 24°C , by default this gives a 3°C hystersis, meaning depening on the programming of the controller it will turn on at 27° or close to it - keep that in mind add realise that it means your 24° might need to be set to 21° so the temp won't go over 24° in your tank.Ok, got it, but what exactly happens now and why does it work?When the controller reads below the set temp, like when setting up a new tank with cold water from the tap, it won't provide mains power on the compressor output.It knows the "fridge" is fine right now.Our added relay will be off for the same reason, however we connected our mians power to the common contact of the new relay, the heater on the NC (normally closed contact) and from there back to neutral.Means our heater will get power when the temp controller in the OFF-state.Now the temp start to go up in your tank and sooner or later it will reach the set temp you adjusted the controller to.Suddelny the controller sees a risk for your food in the fridge to go bad and decides to turn the compressor back on to cool it down.As we added our relay it means it will turn on as well.Our NC, normally closed contact, opens and the heater turns off.From ther is just continues within the range of about 3°C....You added digital temp display to your nice fish tank, don't have to worry anymore about your heater prematurely failing and if you like:Some controller offer an alarm output.This can be used with an added relay the same way as before and would then warm you if you heater burnt out and the temp in your tank goes too low.Why so "complicated" if there is microcontrollers?Anyone can use some Arduino and do the same with ease - if anyone can program what it is required.However, even our average Joe with no electronic skills can to the above mods within a few hours excluding the curing time for the silicone.Back to the basics, and after all we already have a microcontroller inside out firdge controller ;)And as said, if in doubt then even a $10 one will do the trick without any mods to it.What about that stainless steel wire?? Don't tell me I wasted 5 bucks for nothing!You did not...Long heaters or UV lamps certainly benefit from it.But even a smaller one is much more stable inside the pipe if you create a simple wire cage for it.The stuff usually comes in a rolled up form of a 10 or 20 meters.With that it is easy to make some loop with the heater ube in the center.Just make a wrap around the pipe with enough left either end to form a ring or spiral that fits inside the pipe.There is not much spring action happening with this wire, so the loops can be quite big and you just push them inside inside while decreasing the diameter until they slide in.If you have a 3D priter you also just print a ring with a hole for the heater and some spokes to cneter it in the pipe.What about heat transfer and even temps in my tank?With the usual in tank setup you have to ensure anyway that the heater is in the waterflow.Otherwise one side of the tank warms up more than the other.With the heater being in the flow of the external pump directly we pump in warmer water and suck out the colder water the other end.Results in a more stable and even overall temperature especially on long tanks.Enjoy!