A Simple DIY Crab Trap for Catching Aquarium Pests

This DIY crab trap is a quick and easy way to remove unwanted nuisance crabs from your aquarium.

Unwanted pests such as gorilla crabs and stone crabs often make their way into aquariums as hitchhikers on live rock.

At first that my not be a problem. They can even serve as a valuable member of a clean-up crew while they’re small. But as they get bigger and their appetites grow, they will often start to eat corals or other more desirable members of your clean-up crew such as porcelain crabs, snails, and even small fish.

Many aquarists will spend hours trying to spear them with a skewer, or a pair of large tweezers. And some will even resort to spending anywhere from $15 to $50 on a pre-made pest trap.

Before you spend that much time or money though, there is an old reef keepers’ trick you might want to try first.

You can make a very simple DIY crab trap using nothing more than a small glass.

Here you can see the quarter-sized stone crab I caught an ordinary shot glass. Two tries with shrimp didn’t work. But it took less than 30 minutes to catch when I used a little bay scallop as bait.

shot glass crab trap
Small stone crab caught in a DIY shot glass crab trap.

A shot glass will work for small crabs that are about the size of a quarter or smaller.  For larger crabs you can use a bigger glass; one that is just tall enough so that the crab can’t reach the lip if it extends its claw.

The only thing you need other than a glass is some bait. Some people have success with a piece or raw shrimp from the grocery store. I tried shrimp twice but didn’t have much luck with it. On the third attempt to catch the little stone crab that was terrorizing my tank, I tried a small piece of raw bay scallop and it worked like a charm.

Simply place the glass in the bottom of your aquarium about 30 minutes before the lights turn off for the night, and lean it at an angle so the lip of the glass is resting against a rock.

Then place a small piece of the bait in the bottom of the glass and let it sit overnight.

The crab will climb down into the glass to eat the bait. But it won’t be able to get back out because the walls of the glass are too smooth for it climb up.

It make take a few tries, but a lot of reef keepers have had great success using this quick and simple DIY crab trap.


Live Rock – What Type is Best for Your Aquarium?

Live rock is the foundation of a saltwater aquarium. It plays a vital role in keeping your water clean and your aquarium healthy. It provides protection and shelter for your fish and critters. And it gives coral, sponges, and many other things a surface to grow on.

live rock with coraline
Live rock with coraline algae starting to grow on it.

There are several types of live rock to choose from. Let’s take a look at the advantages and disadvantages of some of the more popular options.

Pukani live rock:


  • Very light weight and very porous. Pukani rock provides a lot of surface area for both aerobic and anaerobic bacteria. This means it is very good at helping to remove ammonia, nitrites, and nitrates from the tank.


  • Most dry rocks that came from the ocean have high phosphates. However, many aquarists report Pukinai dry rock has more phosphate than any other type of live rock. High phospates can feed severe outbreaks of nuisance algae.

Tonga live rock:


  • Comes in several unique shapes including branches and shelves.
  • Fairly light weight and porous.


  • Like any dry rock that originally came from the ocean, it can have high levels of phosphate which may cause algae blooms as it leaches out of the rock into your tank water over the first 6-12 months of use.

Man-made live rock:


  • Eco-friendly. You know for certain it is not being harvested from living reefs.
  • Low to no phosphates.
  • Comes in different shapes such as branches and shelves.


  • Less porous than natural pukani or tonga rock.
  • Can be much pricier than other types of rock.

Florida Reef Rock:

Florida reef live rock is limestone rock that is quarried inland from deposits made up of what used to be ancient ocean reefs.


  • Eco-friendly. You know for certain it is not being harvested from living reefs.
  • Florida reef rock from respected sources such as ReefCleaners.org and Marco Rocks are reported to have lower phosphates than most any other type of live rock.
  • Least expensive type of rock.


  • More dense, heavier, and often less porous than Pukani or Tonga rock. So you need to buy a bit more to get the same amount of surface area and biological filtration the other rocks provide.
  • This means more cost, and more weight in your aquarium.
  • If it also lacks very fine pores there will be nowhere for anaerobic bacteria to grow. So, while it may help process ammonia and nitrate, it will not be very effective at reducing nitrates … so some other means of nitrate removal will be needed.

Each type of live rock can be purchased from numerous sources whether it be online or at your local fish store.

And they can be purchased in different ‘states’.

By this I mean you can get pieces that are literally dry chunks of rock (old coral skeletons to be exact) that will need to be cycled and cured.

You can get pieces that have already been cured, cycled, and are shipped or bought wet. This means they are ready to use and have little or no other life on them other than the bacteria needed to consume the harmful ammonia, nitrites, and nitrates produced as waste by the fish and critters in your aquarium.

Or you can get pieces that are aqua-cultured and teaming with life. This means they not only have the need bacteria to act as a biological filter for your aquarium. They will also be covered in anything from barnacles and sponges to coral and crabs.

Each of these ‘states’ of dry rock has advantages and disadvantages as well, but that’s a topic for another article.

Seachem Denitrate: The Best Way to Make It Work

Seachem Denitrate – and Seachem Matrix – can do a fantastic job of helping to lower the nitrates in your aquarium. But there is one very simple, yet important step you must take in order to unlock their full potential.

It took me 6 months to figure this out. And once I did, the impact it had on lowering my nitrates was incredible.

Seachem Denitrate

First, let me point out that Seachem Denitrate and Seachem Matrix are both the same exact media. The only difference is the size of the pieces. Denitrate pieces are about the size of a pea. And Matrix pieces are about the size of a cherry.

The secret to making them work is placing them in the right amount of flow.

The instructions for Denitrate say it should be placed in flow that does not exceed 50 gallons per hour. And the instructions for Matrix do not say anything about flow. (They do hint that it is meant to be used cannister or drip filters, but that’s it.)

Having heard on some forums that Matrix is supposed to be better for lower flow applications, I put some in a media bag and dropped it into my sump.

Over 6 months it hadn’t lowered my nitrates one bit.

So, I did a little more digging and came across a thread on the Seachem support forums that changed everything. It contained a comment from a Seachem rep that said the ideal flow for Denitrate is 20 – 50 gph. And the ideal flow for Matrix is anything over 20 gph.

After spending 6 months in low flow in my sump, my Matrix was covered in crud. I could’ve rinsed it off and re-used. But, being lazy, I decided to just spend $10 on a new bottle of Denitrate instead. (I went with Denitrate because I figured the smaller pieces would provide more surface area for the nitrate-eating bacteria to grow on without taking up any additional space.)

This time though, I put it in a DYI nitrate reactor to make sure it got at least 20 gph of flow (35 gph to be exact).

What a difference!

Within 2 weeks my nitrates fell from 16 ppm to 2 ppm – and have remained there ever since.

It was so amazing and so simple. For more than 8 months I could never get my nitrates to stay below 12 ppm (they usually hovered around 16 ppm). Then, the simple change of giving the Denitrate media a bit more flow completely solved the problem.

And the best part is, it never needs to be replaced. Simply give it a good rinse every couple of months to remove any detritus that gets stuck on its surface and it will work forever.

So that is the key to getting fantastic results with Seachem Denitrate or Matrix: a minimum flow of 20 gph.

Oh, and if you use Denitrate, also be sure the flow does not exceed 50 gph. Otherwise, the water will pass through the media too quickly and the anaerobic bacteria living in its micro-pores will not have time to consume the nitrates.