That is an interesting question.

The entire concept of water column depth (or is that height)in relationship to our passion, hobby, therapy--or whatever it is we call our love of fish is one that really leaves me pleasantly amused and wondering. (Sorry, it is Saturday, I am in office waiting for files to crunch on other puters and so I have too much time on my hands--and am, as a result, in a rambling mood) I mean one obvious answer to the direct question is discus. Another is larger bodied cichlids. Another may be that all fish require deeper tanks if we truly want to see a better stratification of the fish into their "advertised" spots in the tank. By this I am thinking of the description bottom dwellers, mid level and top of the tank fish.

When I think of tank depth, water column and species needs, like everyone else my first thought goes to the where do the fish "hang out". What type of eye appeal is going to be achieved in the tank by the depth and the species. And, I think of the old "rule of thumb" relating to the position of the mouth in relationship to the mid line of the body. And then I think of the twist that in our tanks, given the feeding practices we employ (2x-3x daily--limited amounts...in essence, as a consequence of the feeding pattern, negating the need for some of the practical evolutionary selection of competitive species that has occurred in the wild) that it is not as important that we pay attention. Then, however, I think of the twist that we also introduce by stocking practices and levels.

Russ, presented interesting, educational and fascinating information concerning fish growth during the moderated chat session he chaired. And, by thinking of that info one may come to the realization that tank depth and the stocking and feeding practices encountered in the hobby that the answer goes beyond the traditional angels, discus, large bodied answers to the question. For example if you stock a tank with a number of cories, eartheaters or other bottom feeders,especially in a community of other mid to top level feeders you have introduced a constraint that requires you to pay more attention to tank or water column depth. Think about the shape of the type of fish mentioned. Mouth placed very low on head. Very thin lower jaw. Eyes high on the head--a fair distance above the mouth (at least for a fish). Now go feed the tank and watch the fish eat. You will note that the mid to top level feeders have no real problems plucking food off the surface and out of the water column as it wafts its way to the bottom. Which in a tank full of this type of fish is good. It results in very little food decaying in the substrate creating bacterial disease concerns. Now factor in the bottom feeders.

When these fish are first introduced (as large fry/small juveniles) they tend to fed from the bottom. Over time (sometimes a relatively short period) they become more aggressive at feeding time--coming up in the water column, attempting to get their fair share higher in the tank (it is more than greedy, inconsiderate tankmates--they need more food to grow and thrive). Now look at the problem many bottom feeding species encounter as they climb the water column to feed (many loaches--especially clowns--encluded). They watch the food sinking, they go for a flake, and they miss. They swim right above it, or ram it as they work their jaws attempting to catch and eat it. Because of their basic anatomical structure--eyes high, mouth low, they have difficulty judging the rate of descent and the spot that the food will be in with relationship to the mouth. At a certain point they have difficulty seeing the food as it gets closer to the mouth. They miss--another fish scores (typically the mid level feeders that are built for feeding efficiency at this level of the water column. To offset that problem the fish need to have either more food to select from, or a longer drop time and distance from top to bottom of the water column.

Larger amounts of food result in more food making it to the substrate. Feeding the bottom dwellers better. But, creating competition in their territory for food with mid and top level dwellers/feeders that are also capable of picking the substrate. Result--stress. Consequence--slower growth, greater aggression and more frequent disease occurrence. Depth of tank does relate to the basic physical space required for large bodied fish. It also plays a role in feeding and stocking. I really wish I could define my answer to more closely and directly address your question. I went this way instead to emphasize the need for examining many other consideration that go into the tank and stocking question than simply what body type of fish requires a greater depth. Depth is not necessarily a single, isolated concern. Rather, like most questions is really related to many other variables that have to be considered when stocking the tank and considering species.

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