Introduction: How to Awesome-ize Your HOB Power Filter With Just a Plastic Cup

Hi everyone, my name is Roger and I love this website. Long-time reader but first-time contributer, so please leave plenty of questions, comments, and feedback at the end :)

I recently started keeping freshwater fish in an aquarium which came with an Aquaclear 30 Hang-on-Back (HOB) power filter. While the filter works fine, I noticed that a lot of the clean water that flowed out went straight back into the intake tube to the left (I keep the set the flow to low since I have slow-moving fish.) So basically, the HOB would continually refilter the same small amount of water while leaving the rest of the tank water uncirculated and dirty.

In my mind, there were two possible approaches to this problem. I could shift the filter to the right of the tank and extend the intake tube horizontally to the left with some PVC joints and tubing. Or, I could move the filter to the left and make something to divert the clean water flow to the right, away from the intake tube. Fortunately, the decision was made for me: moving it to the right would've covered some light switches, so... "to the left, to the left!"

This project is quite simple and offensively inexpensive. By the end you should have an efficient, barely noticeable trough that funnels the water away from the intake tube, thereby circulating the aquarium water. Your fish will love you for it!

Here's some photos of a prototype I made. You probably already have a sense of where this project is going just by looking at it:

Step 1: Things You Will Need

1. HOB filter that works
2. Tape measurer
3. Sharpie Pen
4. Hobby knife
5. Shears or strong scissors (depending on how thick your cup material is.)
6. Smaller scissors for more precise cuts
7. A good pair of eyeballs (preferably your own) for eyeballin' measurements
8. Plastic cup (several for trial-and-error)

There is a mindboggling variety of plastic cups in the world, so which one is right for this project? It's really your call.

In general, what you're looking for is a relatively inconspicious cup so it won't detract from the look of the tank (unless you want a 7-Eleven Big Gulp monstrosity to be the focal point of your aquarium!) Clear plastic is preferred since it's durable and see-through. As for size and thickness, it depends on your filter. Mine is just a wimpy Aquaclear 30 (the second weakest of all Aquaclear HOBs), so I did not have to worry too much about the water flow overpowering and bending the cup as it diverts water. If your filter has a greater output, then you should look for a sturdier cup.

The cup material should not be brittle since we'll be bending and contorting it to stay in place under the HOB. It should also be tall enough to catch the whole "waterfall" of clean water from the filter (mine was 3 1/4 inches.) It would also be helpful if the cup has a reinforced lip to bear the weight of the water. Oh, and remember to clean the cup thoroughly so you don't infect your fish with cooties or alcohol poisoning :)

I know, all this fuss over a stupid cup. Don't worry, it'll all become clearer as the project progresses. I just happened to find a stack cheap ones under the sink. They're kind of like the ones your dentist gives you to gargle with after invading your mouth. Perfect.

Step 2: The First Incision

Using the cutter/shear/scissors/cleaver or whatever you're comfortable with, make a straight cut starting from the lip of the cup. Do not cut all the way to the bottom. Instead, leave some room as the bottom of the cup will prevent clean water from leaving back to the intake tube (exactly the thing we are trying to remedy in the first place!) Leaving some room will also preserve more structural integrity in the cup.

Yes, the cup has integrity. Respect that.

Step 3: Follow the Circle

Next, take out your knife. Lay the cup flat on the table with the mouth facing the direction you want the water to flow towards. Then continue the incision from the previous step and cut upwards. Keep going until the length of this new cut reaches the 1/3 point of the circle that forms the bottom of the cup. Basically, the two incisions should form a right-angled L-shaped rift.

Clear as mud? Please consult the pictures. The cup is upside down, but you get the drift:

Step 4: Fold Back the Flap

Now, lift the flap you've made as if you're opening a car trunk. The crease should be parallel to the first incision. If the cup has a reinforced lip which makes it hard to fold, make a small snip at the lip.

This is where the material of the cup becomes important. If the plastic is too thick or inflexible, the flap may just snap off the cup completely. This happened to me and my dentist cup once. You may want to stick some clear or electrical tape along the flap seam to minimize the stress on the material. The tape won't touch any water, so don't worry about contaminating your aquarium.

If you're still reading, that means your cup survived! Celebrate with a cigar and resume work when ready. Because the flap needs to rest straight against the back of the aquarium, make three more cuts along the lip of the flap so it can straighten out.

Step 5: Testing the Fit

Now, the cup and the tank shall meet for the first time. Rest the cup's flap crease directly against the inner edge of the top of the tank. Angle it so that the clean water from the HOB will flow into the cup. At this point you should check to see if the cup touches the aquarium water surface. If it does, you may want to go back to step 3 and cut past the 1/3 point of the circle so that less of your cup i hanging low and too close to the water surface. I would not recommend cutting past the halfway point since that might make what's left of your cup too flimsy to support the weight of the clean water flow.

Is it alright for the cup to touch or be submerged in the water? Sure. But consider two things:

1. The cup will be more noticeable to you and your fish.
2. The clean water won't hit the surface as hard, thus decreasing the rate of gas exchange and the overall levels of oxygen in the water for your fish.

Honestly, fish aren't known to be concerned with tank aesthetics. And you may have an air pump that ensures gas exchange. So it's all good :)

Back to the task at hand, see where the flap folds over the tank edge? Mark it with a sharpie and make another snip in the lip there. Then push the flap down with some force so that it makes a crease. By now your cup should resemble the HOB in the way it hangs off the tank, except it's on the inside.

Step 6: Here's Comes the Tricky Part..

So this is the part where you will need your eyeballing and guesstimating skills.

Put on your guesstimating hat and put the bottom of the cup against the side of the HOB. Assuming the cup is clear, look through the bottom and observe the shape of the HOB part where the "waterfall" of clean water comes out. Basically, we want to carefully cut the profile shape of this part into the bottom of the cup so that the HOB mouth will slot neatly into place. This will minimize water leakage towards the intake tube and effectively divert all the clean water that comes out.

A picture's worth a thousand words, so take a look.

It's a bit hard to make out, but what I've done is I've used the sharpie to color in the area where the HOB protrudes into the cup. Then, I cut out the colored part and try the fit again. Doesn't fit? Cut a bit more. Still doesn't fit? Curse and cut some more.

Really, it's a trial-and-error process. Since every filter is shaped differently, you're going to have to be patient and cut a little bit at a time. You may be tempted to just make a big hole like I did with one prototype, but then you'll find out what I learned first-hand: the hole leaks water to the intake tube and it makes the cup too weak to support the clean waterfall. So endure the frustration, chip away at it, and you'll end up with a snug and custom fit between the cup and the HOB.

TIP: A lot of HOB waters have a nearly vertical edge where the water drops off. You can make your cup fit really well to this by cutting a vertical slot to match this edge.

Step 7: Success?

Don't worry if you accidentally cut out too much or if the cup implodes. That's why we got multiple cups before we started, right? RIGHT?! ;)

When you're done, it should look something like this...

You'll probably find that the cup flap no longer fits exactly to edge as you fiddle around with the cup. This is fine. The cup is held in place by the weight of the HOB which pins the flap against the edge, so whether the creases of the flap match the edge of the tank is fairly irrelevant. You can rebend the creases to fit better if you're a perfectionist. And you can tape the flap to the back of the aquarium if you're paranoid and want extra stability

Step 8: You Show Me Yours, I'll Show You Mine

Step 9: Level the Top

Almost there! The last finishing touch is to cut any part of the cup that sticks out above the aquarium so we can fit the hood back on. You can either use the sides of the tank or the hood itself as a reference for how high your cup is allowed to be. Cut off as little as possible; the cup's strength comes from retaining as much of the original circle as possible.

Step 10: Ta-da! Waterflow Diverted!

Perform any final adjustments so that the cup sits still and at the angle you want. Pop on the hood, prime the intake tube, and let the filter live! If everything's went according to plan, the HOB should pour out clean water into the cup which then collects and diverts the flow away from the intake tube! Make sure the cup catches all of the waterfall; if there's any leaks out of the bottom of the cup and falls towards the intake tube, push the cup close to the intake. If the waterflow is so powerful that it rolls straight off the cup instead of being diverted, you may need to reduce the strength of the flow if you HOB has that function. If your cup flattens out under the weight of the water then you may need to start over again with more conservative cuts and angles, or with a different and stronger cup.

So, what do you think? I made mine about an hour ago so I can't speak to the long-term benefits of this set-up. However, I do notice that no water is being refiltered endlessly and the water in the tank is circulating better. As an added bonus, I can now turn up the waterflow to max since the cup seems to soften the the stream somewhat while maintaining the water surface agitation that's needed for gas exchange! So now there's more filtration without exhausting slow-moving fish!

Some improvements are possible. We could try taller cups, like the kind you get for blended frappaccinos at Starbucks or tapioca bubble tea drinks. This would extend the range of the clean water. I imagine a water bottle would work well too. You could make holes along the length of the bottle so that clean water will fall out of the holes and onto the surface like rain. If the cup or bottle can support it, you can even place gravel or some other kind of filter media to add extra biological filtration to your aquarium.

So all in all, I'd say this project has been a great success and has improved the overall health of the aquarium ecosystem. Thanks for reading my first instructable, I hope you enjoyed it. Best of luck to those who are keen on trying this; may you and your fish live happily ever after :)