Q. I am a new reader of your magazine, and I am quite satisfied with it. As any other reader, however, I have several questions regarding the up keep of my marine aquarium. I have a 65-gallon tank with a canister filter, and I am about to install a pair of powerheads on the undergravel filter. My intention is to use one of powerheads in the normal flow, and, because it is very powerful and feature reverse flow capability, use the second one in reverse.

This is because such as setup supposedly strengthens existing bacteria populations in the gravel bed and also stimulates a bacteria colony under the gravel plate. I talked to my dealer about it. Despite his experience, he said he has never done it, but maybe it would be worth a try. I have presently five small and one medium fish, and I intend to add two or three small ones. What do you think?

A. This is a good question. Reverse-flow filtration was becoming quite popular before the wet-dry filter came on the scene. The advantages of setting up a filter in the reverse flow manner is that it enhances biological filtration.

With a conventional undergravel filter, only the upper surface of the substrate develops a rich nitrifying bacteria colony. But, by pushing oxygenated water under the filter bed, the lower surface of the substrate will develop a healthy growth as well. The growth of bacteria on the upper surfaces of the gravel bed is a result of its exposure to the oxygenated water that is moving about the interior of the tank. Another advantage to encouraging nitrifying bacteria growth on the substrate surface nearest to the undergravel plate is that these bacteria will not have to compete for growing surfaces with microalgae because light does not reach the lower regions of the gravel bed.

One thing you will need to do if you are going to set-up a reverse-flow undergravel filter is to pre-filter the water of larger particulates before you pump it under the undergravel plate. To do this you can attach a sponge filter to the intake opening of the powerhead, or you can use a canister filter to pump filtered water down the undergravel uplift tube. If you don't filter the water before pushing it under the filter plate, the plate is more likely to get clogged by larger particulates. Water will then be channeled through the areas where the filter bed is not clogged.

One way to ensure that the water is flowing evenly through the entire filter bed is to clean the filter bed regularly with a gravel vacuum. With an undergravel filter is configured in a reverse-flow setup, the filter bed should not clog as readily as it does in the conventional configuration. The pre-filtered water being pushed under the undergravel plate will minimize this problem.

The other thing to consider before you set up your filter is the configuration of the undergravel plates themselves. If the tank has one large plate, then you will want to set both powerheads so that they are pushing water under the filter plate. If there is only one plate, and water is being pushed down (reverse flow) one lift tube and being pulled up a second lift tube in the normal way, the water that is pushed under the plate by one powerhead is going to be pumped out from under the plate by the other one. The water will neither be pushed up or pulled down through the substrate, and the filter will not work effectively!

Porcupinefish
Q. I am a enthusiastic reader of AFI, living in Panama. The reason why I'm sending this letter is for a little help. Last Sunday I caught a Diodon hystrix (usually called "pez globo" in my country) on the Pacific Coast of Panama. I want to know if I can have this fish in my tank.

I have a 55-gallon tank with six damselfishes and two clownfishes. I have a wet/dry filter and two outside power filters on the tank. I also change 5 gallons of saltwater every month. Finally, there is 80 watts of fluorescent light over the tank.

Can I have this fish with my damselishes? If the answer is yes, can you give me some general information about this fish? If it's not, which fish do you think I can keep with the damselfishes?

I need this information from you because in Panama there isn't any written information about this combination of fish available. And in aquarium stores people tell me different things. So I decided to ask you, because you have a lot of experience in these matters.

A. The porcupinefishes of the genus Doiodon are very popular in the U.S. aquarium trade. However, the species you have collected, Diodon hystrix, is not commonly seen in fish stores here. This species is often referred to as the black-spotted porcupinefish. The species most commonly encountered by aquarists in the U.S. is the ballonfish or fine-spotted porcupinefish (Diodon holacanthus).

The biggest difference in these two species is the sizes they can attain. The ballonfish attains a maximum length of 20 inches in the wild, but rarely exceeds 12 inches in captivity. On the other hand, D. hystrix is said to reach a whopping 36 inches in total length, and commonly reaches 24 inches in the aquarium! So, you will either have to get a larger aquarium or release the fish when it outgrows your 55-gallon tank.

The second option means finding a home for an oversized fish, which can present many problems. If your porcupinefish is kept with fish from other regions, it may be exposed to a parasite or pathogen not indigenous to the waters around Panama. To then release it in your area could be catastrophic to local fish stocks.

Diodon hystrix is a durable fish that will even withstand most of the mistakes heaped upon it by the inexperienced or careless aquarist. They will come down with white spot — saltwater ich — (Cryptocaryon irritans) quite frequently, and can even harbor this parasite in their internal organs. When this parasites is in its free-swimming stage (which occurs after the cysts drop-off of the body and hatch) it can be destroyed with copper-based medications.

This fish is often quite shy when first introduced to the tank, and thus should be provided with a cave or overhang that it can hide out in or under. Although they can be quite reclusive initially, with time this fish will become very tame, even taking food from the fingers of its caretaker (be careful if you try and hand feed your pet, as its powerful jaws can inflict some damage).

The porcupinefish is nocturnal and feeds on hard-shelled invertebrates, including sea urchins, snails, hermit crabs and bivalves. It is a threat to all these invertebrates and many more (including shrimps, corals and anemones) in the home aquarium, making it unsuitable for reef systems.

Although they typically do not eat fish in the wild, in captivity they will occasionally nip the fins of their tankmates (especially those with long, flowing finnage, like the batfishes — family Ephippidae), and will even ingest smaller fish if they can catch them! Although your damsels will probably be able to avoid the beak-like jaws of the porcupinefish if there are plenty of hiding places in the tank, you should recognize that they are a potential meal for this fish.

Feed this fish a varied diet that includes fresh crab, shrimp, clam and squid, and make sure to include some hard-shelled prey items in its diet from time-to-time to help wear down its ever-growing teeth. If you fail to do this, the teeth can get so long that it may have problems ingesting food.

Of course, the behavior that the porcupinefishes are renowned for is their ability to inflate with water or air. They usually engage in this practice when they are threatened by a predator. In the genus Diodon the spines that lie alongside the body are erected when their abdomen is inflated, while in the genus Chilomycterus, whose members are often referred to as spiny boxfishes, the spines are erect all the time.

Because of their propensity to inflate when threatened, it is important not to lift you porcupinefish out of the water at any time. If it should inflate with air, sometimes it is unable to expel the air and will float helplessly around the tank until it perishes.

Although an inflated porcupinefish would not appear to be a very appetizing meal for a predator, there are several shark species that eat them regularly! In particular, the tiger shark (Galeocerdo cuvier) and the Galapagos shark (Carcharhinus galapagensis) find an inflated porcupine fish to be just fine as dinner.