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I have a goal tank, for when I get my fish room. This is not a current or near-future project, but I like to plan ahead, occasionally way ahead. Unfortunately, it's hard to plan something like this alone, hence why I'm looking for recommendations from such experienced persons as can be found here on planetcatfish.

This would be a custom tank, possibly built into the wall.
it's intended occupants would be three to four bullheads, Ameiurus melas and Ameiurus natalis. Once the cats had attained sufficient size, the true centerpiece would be added, a bowfin, Amia calva. Panfish, bluegill and crappie, may or may not be added. You may have noticed this is to be a native species tank, all the fish can be caught locally.

I believe that a tank 72"x36"x27" would hold these fish in a comfortable 300 gallons, or do I need to rethink the size? I hesitate to make it to much wider or taller for cleaning purposes, but It's the fish's comfort that counts.
I only have experience with glass aquariums, though I know some people who swear by Plexiglas. Is Plexiglas an option for such a large tank? Would a sump filter would appear to be best for a tank this size? I wouldn't need a heater, but is there other equipment to be considered?

Occupants of the tank.
I have no experience in keeping native American fish, so I can't comment on that.

The dimensions you suggest allows for fish up to about 15-18" max size.

Filtration.
The options for filtration on large tanks are:
1. Sump.
The "obvious" choice. Allows for LARGE filter volume, and adds extra volume. Also allows for hiding the heater and other such items - not that you care about heaters.

2. "Pond" filtration system.
Essentially the same as a sump, but usually the return feed is gravity and the water is pumped INTO the filter container. Again, allows for large amounts of filter media of many different types. The main difference between pond and sump filtration is the location of the filter container itself - pond filter needs to be placed ABOVE the tank, sump is below. [Of course, one can often make a pond filter into a sump and vice versa by moving the pump around].

3. External canister filters.
There are some external canister filters that can be used for very large tanks. Smaller filter volume, but allows for multiple units. Doesn't add as much extra volume as the pond/sump solutions.

4. Hamburg matt filter.
With this solution, the side or back of the tank is used as a filter canister. The benefit of this is that ALL of the "stuff" needed for the tank can be kept inside the tank itself. A compartment at the back/side holds the filter media, and a pump transfers water from inside the compartment into the main tank, and the filtration happens because the water flows back in through the filter media. Drawback is that it adds absolutely nothing to the overall volume of the tank, so whatever size tank it is, will be the actual volume of water...

All filters have their own strengths and weaknesses. For a large system like this, it's probably worth considering doubling up on "single point of failure items, such as having two pumps in a sump, or using an external canister filter and a sump filter, etc.

Tank material.
There are good and bad things about acrylic (perspex, Plexiglas) and glass respectively. The acrylic is lighter, but scratches more easily - you probably don't want to keep plecos in an acrylic tank - they will eventually scratch the surface of the tank so that it is no longer perfectly clear.

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Mats

what would be the necessary dimensions for a fish that gets in the 25-30"? the bowfin is more of an ambush style predator than a cruiser.

would 92x42x36 be more appropriate?

I'm fond of running more than one kind of filter on a tank, the sump & canister sounds like a good combination.

I was thinking of acrylic because it seems a bit less breakable, is it possible to have a tiled surface for the back, sides, and floor with only the front being acrylic? I was thinking a natural stone tile.

Well, my personal guideline for any fish is that you should have AT LEAST 4L x 2L x 2L tank size, where L is the length of the fish, so a 25" fish would require a tank that is 100" x 50" x 50" - you can usually "cheat" a bit on the height when it's quite a tall tank in the first place.

Of course, other people will have other "rules" for how large a tank needs to be, in both directions. Someone suggested that the tank should be 16 times longer than the fish - now, for a 25" fish that makes a 400" long...

I don't think there is a huge difference in "breakability" of acrylic vs. glass when at the proper design size - obviously, if the material is too think for the size of the tank, it's likely to break.

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Mats

Oh, yes, and if you just use the right kind of "sticky stuff" (for example aquarium silicon), you can certainly stick some sort of natural stone tiles on the side and back of the tank.

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Mats

Well, the tiles wouldn't be on the back of the aquarium, they would be the back, but i think we're just using different words o describe the same idea.

well, that would make my dimensions a hair short. A longer tank isn't a problem, but width is another story. I'm about 5'3", how the heck do I reach the back of a 4' wide tank for cleaning and such? granted, I would use a fine round gravel for this application, so I would have the length of the gravel vac, but that still seems like quite a reach.

I think if you went with that width, you'd have to do what the public aquariums do, which is to climb right in.

I'm not 100% sure we mean the same thing. Yes, it's possible to build an aquarium where three sides are not glass. It's done by public aquariums quite often. It is not EASY to do that, and if you just want to have a stone "cover" on the inside of the tank, then it's probably better to use the method of sticking a cover on the glass.

Public aquariums often use steel reinforced concrete, and I have read about tanks built from wood/plywood too. The trick is to understand how to make it strong enough to hold the water well. It is much less hassle to buy a ready made tank of the size you want/need. For very large tanks, they are usually assembled in place.

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Mats

I see. I honestly don't know a thing about construction, so I will take your advice and scrap the only acrylic front idea.

I really don't want to climb in to a tank with a bowfin in it, they are infamous for big teeth and bad attitudes, and bullhead stings are painful too. I would probably have to skimp on the width a bit and use a step stool.

Are you on monsterfishkeepers.com? Plenty of humungous tanks there. Many people with > 1000 gallons. One guy is doing a 4300 plywood tank right now. I would say once you get over 240 gallons, the plywood tanks make way more sense.

plywood? for reinforcing it?

I haven't tried over at MFK yet, though perhaps I will.

Plywood for the framing and structural support, and then fiberglass, zavlar, various pond liners to form the tank with a piece of glass in front. If you're considering that huge a tank, MFK is a great resource.