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How to convert a Tetra EX-20 power filter to an HOB algae scrubber


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Finally, DIY for people who have been wanting a hang-on-back algae scrubber: here are the plans for converting a Tetra EX-20 power filter to a HOB upflow scrubber!






Note: This very simple DIY version is not quiet. If made for a nano, it will probably be the loudest thing on your setup. It is made to be an easy DIY by not including extra steps which would make it more quiet, although it will get more quiet as it fills with growth. These optional steps for quieting will be mentioned but not shown. This simple DIY however does keep all the red light inside; none gets into the tank, or on the glass, and only very small red specks of red light are visible on the top.


First, here is the filter you need to buy:






Other power filters will work too, you will just need to change the shape of the wall in the next step.


Now, take out all the internal cartridge pieces; none of them are needed anymore. The green frame (which is meant to hold the filter bag) is already shaped to match the curve of the filter box, so we will trace it’s shape onto a thin piece of black styrene plastic that you can buy at any hobby or craft shop. The green frame has two halves; one of them is slightly larger than the other; use this larger one for tracing:






Now cut a flat section out of the curve; this will be at the bottom of the filter box, and will let water go under it (it is upside down here)…






Now apply your algae-attachment material. Any type of epoxy and aquarium gravel will work, but Devcon 30 Minute is available most places; and the whiter the gravel, the better, so that it reflects more light to the growth. You could also used roughed up plastic canvas:






Next you will need an air line clip, and a “T” connector; the one shown is a double clip (holds 2 hoses), but a single clip is good too:








Now rotate it so the curve is on the bottom, and epoxy the air line clip at the bottom:






Try putting the “T” connector into the air line clip, and see how it fits; then remove it:






To make installation easier, you can remove the power cord and water pump; this is not necessary, but makes it easier to set the filter box on your table:






Now for the hardest part: glue the plastic wall (with gravel or screen material) into the filter box. But first, test-fit the wall for a good fit, to make sure it has almost no gaps. Push the plastic wall up against the internal rails of the filter box. These are the rails that the green filter bag frame slides into; use the rail that is closest to the front of the filter box, and put the plastic wall on the front side of this rail. Note that the top of the plastic wall should be at the top of the other wall that is already in the filter box. If the fit it good, epoxy away!…








Now drill a hole about 3/8” (10mm) on the right side of the filter box, at the top, for the air tubing to go out; attach the “T” connector to the air line clip, and run the air tubing through the drilled hole. Note that the hole must go under the rim of the filter box, so that the top of the filter box can still snap on, and so that internal water splashing will be kept inside:








The pics show the hole on the right side, but you may want to put the hole on the left side instead, so that the tubing can go the same path as the filter power cord. Inside the filter box, the air tubing should go down first, to the bottom of the filter box, and then upwards to the hole. This way the tubing will not be in your way when you reach in for harvesting.






The LED that you use should be red, 660nm or maybe 630nm, and can be 1 watt or 3 watt. If your nutrients measure low or zero, use the 1 watt; if your nitrate is above 20, or phosphate above 0.2, then use 3 watt. Use a waterproof LED (very hard to find), or just dip a regular LED in epoxy:






To dip your own LED, first solder your wire to the LED. Then clean the solder flux off, using alcohol and a toothbrush; then clean again with fresh alcohol to get all residue off of the LED and the first 6” of wire. Then dip the LED and the first ½” of wire into clear epoxy such as Devcon 30 Minute, and let harden over night. Then dip again in new epoxy, but this time go further up the wire, about 2”. Let dry over night again. Now epoxy your LED into position so that it shines on the middle of the algae attachment surface.


ONLY OPERATE THE LED UNDER WATER. It will overheat and melt the filter box if you leave the LED on without water. You can turn the LED on for just a few seconds to make sure it works, but any longer than this needs to be underwater only. After the LED is in place and the wire routed through the hole in the side of the filter box, silicone the hole on the inside and outside, to keep the water inside when it is bubbling.


Now snap the lid on the filter box, and put the water pump back on, and it is ready to use! Like all algae scrubbers, start at 18 hours/day of light, but run the water pump 24/7. Also, if you don’t see some brown growth within a week or so, you may need to shade the LED so it is not too bright; this can be done with smaller pieces of black plastic put inside the filter box. And now, final pics:










Air bubbles will go up the LED-side of the plastic wall you put in, and water will go down and under the wall, so that no bubbles get out. Almost all red light will be blocked from getting into the aquarium because the black surfaces won’t reflect it. And harvesting (cleaning) is done simply and easily by just lifting the hinged lid; no need to take anything apart. Here is an overview video:



Variations: The first mods that you may want to consider are ones to make the scrubber more quiet. You can always try it as-is first, and add the mods later if needed. The way it currently is, the sound of the bubbles comes through the walls of the plastic filter box. Of course you can reduce air flow, such as at night if it’s near your bed, but the best results are always with more bubbles, not less. You could also use an aquarium airstone, which would make smaller bubbles and thus be more quiet, but an airstone will need periodic cleaning when it clogs with growth.


The first way to reduce the bubble sound is to thicken the walls by adding epoxy to them, or maybe, epoxy and gravel. To do this, you would coat the inside of the entire compartment with epoxy, and maybe even do so more than once. The thicker, the better (weight does not matter).


You could also try doing this to the lid, but you will need to make sure the lid can still fit. One way to try this is to use reef-putty: put a thin layer all over the bottom of the lid, then lay a sheet of plastic sandwich wrap over it, and press it into position on the filter box; when it is dry, pull the plastic wrap off and the putty will have taken the shape of the internal parts such as the water tube.


Next you could add some sound-absorbing material to the inside of the lid; this well help reduce sound from reflecting down through the water drain area into the aquarium. Black polyester felt, maybe 1/8” thick, glued to the bottom of the entire top part all the way to the end of the water path, will absorb a lot of the reflecting sound inside. Just make sure the lid fits before gluing anything in place.


Lastly you could try to seal the crack where the lid opens up. One way is to put some black silicone along the crack, and then lay a sheet of sandwich plastic wrap over the opening, and then close the lid. When the silicone dries, remove the plastic wrap and the lid should fit tight with no gaps.


So, post your build and growth pics here, so we can all see how it is going!


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I'd be super interested to see how the harvesting is done with this. Maybe I read over it too quickly but it seems like it would be difficult to me.

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Once it is growing thick, just lift up the hatch and reach in and pull the algae out.


If it's slimy or dark growth, then just take the whole thing to the sink and use a toothbrush.

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