20 Gallon Hex Tank Stocking

Hi everyone! 
I recently jumped on a deal for a 20 gallon hexagonal tank but didn't really think about what I was going to put in it. Because of the size I was thinking somewhere along the line of a community tank since it's too small for cichlids or larger fish. Also, because of the shape and size I'm pretty sure that adds a bit more limits. 
I kinda wanted some type of algae eater (sucker-mouth catfish), scavengers (loaches,etc.), and a main attractive fish (platys, rasboras, glass/clear fish etc.). I however have no interest in little tetras or guppies because of past experience. I have some live plants, driftwood, and decor in my larger tank (55 gal.) I could put in this tank as well if required for some species. Any responses are appreciated. Thanks!  

You are correct that the tank does present limits, both by the shape but also the volume.  Small fish are ideally suited to this tank, so I hope we can find some that you like.
 
Water parameters of the tap water are going to be particularly important here, because most of the small fish will be wild caught and thus have rather specific requirements.  If you don't already know the GH (general hardness) and pH, you can ascertain these from the water authority, who probably have a website with data.  The KH (carbonate hardness, or Alkalinity) is also worth knowing as this will tell us if the pH is likely to remain stable or shift a bit.
 
The fish you mention wanting are basically all too large for this tank.  But we can get more into this when we know the water parameters.
 
Byron.

The water pH from the bigger tank is about 8.0 (very high from Texas holey rock), Nitrates ~10-20ppm, Nitrites ~0ppm, Ammonia ~.1-.25ppm. I put some filter media and about 10 gallons of water from my 55 gallon (already cycled of course) into the new tank to speed up the cycling process. I am also running a fluval 206 on my new tank as well. As for stocking options, I'm up for any suggestions at this point.
Thanks! 

With that high of a pH, we are definitely going to need to know the hardness.  Sometimes a high pH equates to a higher GH, but not always.  Hardness is the amount of dissolved mineral salts in water, and it is these that most affect fish.  There are fish that need soft water and have difficulty in hard water, and there are other fish than need the minerals so they are better in hard water.
 
To get the GH, check if your municipal water authority has a website; the data will likely include GH.  If you find it and can't figure out the data, post the link and I'll take a look.  Sometimes this data is difficult to decipher.
 
I really would rather have the GH before I start suggesting fish, so we can be assured that what is suggested will be able to manage.
 
Byron.

I'm not sure if this is what you're looking for but I checked on my local municipal water website and found this recent report. 

Fiji said:

Nitrates ~10-20ppm, Nitrites ~0ppm, Ammonia ~.1-.25ppm.

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Just a suggestion, but I would get that ammonia under control first before starting on a new tank. Any traces of ammonia present in a tank can be detrimental on a fish's overall health and longevity, if they even survive the exposure.
 

Fiji said:

I'm not sure if this is what you're looking for but I checked on my local municipal water website and found this recent report. 

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Since I used to work in the Water Authority I immediately found it funny that the average, minimum and maximum's are all exact, but that is just me (knowing it is impossible to maintain exact readings).
 
Anyways, Byron would be more of the expert on how this equates to your aquarium, but I see Total Hardness is reported at 89.7 ppm (89.7 mg/L or ~5° dH).  I believe this puts you in the "Moderately Soft" to "Slightly Hard" categories (allowing for fluctuation).

Those are interesting numbers.  The KH is 6 dKH (107 ppm), and the GH is 5 dGH (89.7 ppm).  The GH is next to ideal for soft water fish, and most plants.  The TDS at 270 is another important factor.  TDS is basically everything dissolved in the water: chlorine, chloramine, ammonia, phosphate, salt, hard minerals (GH), bicarbonates (KH), etc.  And almost every substance added to the water will increase TDS: water conditioner, fish foods, plant fertilizers, calcareous substances, medications, water adjustment products, etc.  The water of soft water fish is generally close to zero in TDS, while by contrast the water in Lake Tanganyika is around 400.  The KH buffers the pH, so it may not lower a great deal.  Lots of wood and dried leaves are good aqquascaping elements that will work to lower TDS and pH.
 
Byron.

 So based on this information, what category of fish do you have in mind that I could stock my tank with? I have never worried about water hardness in the past so I know next to nothing about how it affects the aquarium.

Fiji said:

 So based on this information, what category of fish do you have in mind that I could stock my tank with? I have never worried about water hardness in the past so I know next to nothing about how it affects the aquarium.

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I purposely held off answering this previously, as I wanted to think about it.  I would want to see the pH down to around 7, as I would go with fish more attuned to soft water than hard water.  I say this because your GH is very low for hard water fish.  There are fewer of them suited to small spaces, so that leans me toward soft water fish again.
 
So, using your post #1 fish as a starting point, I would forget livebearers (platy, etc) as they need the mineral to be in their best health.  And they are not small fish anyway.  Rasboras would be better, being soft water fish, and another advantage is that most of them are fairly sedate, the opposite of active swimmers like the barbs and danios for example.  In confined spaces, sedate fish obviously work better.  The three species in Trigonostigma are obvious choices: the common Harlequin Rasbora and its two very close relatives T. espei and T. hengeli which would actually be even better, my personal favourite of which is T. hengeli (you can google these names to see photos).  Then there are the "dwarf" rasbora species in the genus Boraras which have become more often seen as they suit nano-type tanks so well.
 
You mentioned scavenegers and loaches.  There are no loaches suited to this small a space, as they are shoaling fish needing numbers and more room, even the dwarf loach and dwarf banded loach.  Kuhlii loach in a group of say five could work, with sand substrate, but you will not see them much.  The corys offer more options.  The three "dwarf" species certainly, in groups of 10-12, and some of the medium sized corys can work like the panda, in a group of five-six.
 
Algae-eater and suckermouth...do you want these because you like them as fish, or to handle algae?  I ask because many starting out think they need an "algae eating" fish but that isn't always wise thinking.  But one fish that comes to mind that would be interesting and have some benefits on algae is the common Whiptail, Rineloricaria parva.  There is also the "red" form usually sold as "Red Lizard" or some such silly name, but it is different.  I have both and must say I see a lot more of the common Whiptail, always out and about, than I do the reds.
 
Some other options are the pencilfish.  Nannostomus eques (a group of 7-9) is good in this sort of tank, and their oblique swimming posture can add interest.  The Ember Tetra, Hyphessobrycon amandae, for some bright orange-red colour, in a group of 9-10.
 
Obviously not all of the afore-mentioned, but this should give you some ideas.
 
And I concur with BiggTex on the ammonia...it is very low, but I wonder what it's from?
 
Byron.

I've never previously heard of pencilfish but they look/sound pretty interesting! Rasboras also look pretty attractive but they're really tiny fish. The whiptail catfish would probably be a good algae eater for me but isn't it a bit on the large side for my tank? I've had a BN pleco in my larger tank for ~2 years and its only about 3-4". And I'm not sure the whiptails and pencilfish are available at my local fish store so I'd need to buy them online.
 I've taken Kuhli loaches into consideration but I've heard they only come out at night. I've never owned corys before but I have heard really good/funny stories about them.
The water measurements and ammonia levels are from the water I used to fill my new tank from my bigger tank, not the current water parameters. Also take into account that I have a Fluval 206 running which will create quite a current to the fish. Thanks! 
 

I've never previously heard of pencilfish but they look/sound pretty interesting! Rasboras also look pretty attractive but they're really tiny fish. The whiptail catfish would probably be a good algae eater for me but isn't it a bit on the large side for my tank? I've had a BN pleco in my larger tank for ~2 years and its only about 3-4". And I'm not sure the whiptails and pencilfish are available at my local fish store so I'd need to buy them online.
 

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With small tanks, say 30 gallons and under, the "dwarf" sized fish work better because you can have more of them and they "fill" the space so it seems larger.  I don't know which "rasbora" you are thinking of; the Boraras species are small, about 1/2 inch, but their bright red colouration means that in a group of 9-12 they can be very nice.  They tend to spread out around the tank too, more than the larger Trigonostigma species I mentioned which tend to remain in a group and normally do not swim all that much.
 
The length of the common whiptail at 5-6 inches may seem "large" but this fish is so narrow it has a very small impact on the bioload.  And it too is not active swimming, so it adds some interest, performs some algae cleaning.  A single Bristlenose would be OK, though personally I would not have one of these in a 20g (or smaller) tank.  You have to be careful in small tanks; any largish fish  (and the BN has the mass which the Whiptail does not) will immediately make the tank seem small, and dwarf the other fish.
 
Byron.

Byron said:

 

I've never previously heard of pencilfish but they look/sound pretty interesting! Rasboras also look pretty attractive but they're really tiny fish. The whiptail catfish would probably be a good algae eater for me but isn't it a bit on the large side for my tank? I've had a BN pleco in my larger tank for ~2 years and its only about 3-4". And I'm not sure the whiptails and pencilfish are available at my local fish store so I'd need to buy them online.
 

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With small tanks, say 30 gallons and under, the "dwarf" sized fish work better because you can have more of them and they "fill" the space so it seems larger.  I don't know which "rasbora" you are thinking of; the Boraras species are small, about 1/2 inch, but their bright red colouration means that in a group of 9-12 they can be very nice.  They tend to spread out around the tank too, more than the larger Trigonostigma species I mentioned which tend to remain in a group and normally do not swim all that much.
 
The length of the common whiptail at 5-6 inches may seem "large" but this fish is so narrow it has a very small impact on the bioload.  And it too is not active swimming, so it adds some interest, performs some algae cleaning.  A single Bristlenose would be OK, though personally I would not have one of these in a 20g (or smaller) tank.  You have to be careful in small tanks; any largish fish  (and the BN has the mass which the Whiptail does not) will immediately make the tank seem small, and dwarf the other fish.
 
Byron.
 

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I did some research and am defiantly interested in the whiptail catfish and probably some Trigonostigmas. Thanks!