Some newbie questions about FW tanks.

Both eels and oscars are too big for that tank.

Saltwater is generally more expensive. The substrate and the livestock are pricier than freshwater, and water changes are more expensive because you have to use salt.

I'd recommend waiting on the saltwater and trying freshwater first....get really comfortable with that before you move on.

The price depends on what you want to put in the tank. Are you including the tank and a stand in the estimate?

If you get high quality equipment it is going to cost more, but do you better in the long run. Different types of filters are also more or less expensive, though one is not necessarily better than the next.

I'm not sure if you're asking what fish produce less mess, or if there's a fish you can add to a currently stocked tank that will reduce the need for water changes.
No, there is no fish or invert that will reduce the number of water changes. There are animals that will clean up uneaten food, but if you're not overfeeding then it's not an issue.

If you're asking what fish are less messy-usually small fish produce less waste. Certain types of fish, like goldfish and oscars, produce a LOT of waste for their size.

OSCAR = Bigger Tank

Skip the SW and use the extra cash to by a big tank (55+) for that Oscar you want.

Short estimate although I would encourage you to be patient and watch craigslist for a screaming deal...

Tank = All-Glass 75 G bought at LFS ~$100
Stand = Build it yourself ~ $50 material
Filters = HOT Magnum Canister (~ $50) and Emperor 400 HOB ( ~$45) at Petsmart > take a printout of the online price as the store price is much higher. They will match their online price.
Glass Top = ~$35 drsfostersmith.com
Strip Light = ~$100 drsfostersmith.com
Heaters = Two (in case one breaks or sticks in the on setting) @ ~$18 each drsfostersmith.com
Misc = GOOD WATER TEST KIT, Plants, Substrate, Buckets(or Python = +$20), Net, Water Conditioner, Thermometer ~$125

http://www.aquariacentral.com/forums/showthread.php?t=42633

Try that posting first.

You could get a decent setup for around $100 or a deluxed setup for under $225 (including Perfecto wood stand, strip light and canopy). I would run a FW setup for a little while before considering SW. If you're only wanting a 29 gallon tank, Oscars are out and your only eel options would be a Peacock or ZigZag eel. If you get one, you'll think it's the coolest thing EVER when you first put it in. Then it'll dive under the substrate and you won't see it unless you tick it off cleaning the tank.

My advice: keep looking online and at your LFS until you have a strong preference on fish that will do well in your tank. I have a loach tank, and my son has a primarily catfish tank. I recently added a few Cichlids to my son's tank, and I am convinced they are the coolest fish. They recognize us when we come into the room and they dart about like a dog does when he's happy you're home.

You can have fish or inverts who scavenge, but nothing will replace water changes. I take out at least 25% each week, and in my son's overcrowded catfish tank, I take out 50% each week. If you are disciplined in stocking your tank and in testing your water, you could probably get away with every other week.

It doesn't seem like much, but when you have everything up and running, you'll have a deeper appreciation for Creation and a relaxing hobby.

You obviously can't keep the oscars in a tank that small, same goes for the spiny eels, but if you want a cichlid with personality, you can try perhaps a blue acara or another one of the small and midsize acaras, like Bujurquina sp. and Krobia sp.

SW appears more intimidating.but it really isn't ..it is more expensive.

but you can set up a nano tank fairly cheap.
the only thing that seems more expensive it what appears to be cost..of mixing water..but it really isn't..in SW tanks you don't need to change as much water 10% is fairly common as a weekly change..
if set up correctly..SW tanks have a nearly complete nitrogen cycle...meaning the nitrate is processed..the water changes remove tds and re add nutrients lost thru consumption by the inhabitants.
the cost can be higher as many use live rock which runs $3-$6 per pound you need approx 1-1.5 # per gallon.

but this is what hosts bacteria which consumes ammonia, nitrites and nitrates.

in the long run it winds up costing about the same since you don't use any media and it is self replenishing.

you can run a skimmer but when the tank is stable the skimmer isn't needed.
other than that you can run a HOB filled with LR rubble.

here's the interesting part. to cycle a SW tank you toss in a bunch of LR uncured but cleaned into a tank filled with ro water and specific gravity of 1.023- 1.025. run the filter(15X turn over rate) and wait..in about 3-4 weeks the tank is cycled..change about 30-50% water and add fish. tossin some inverts to keep the tank clean.
a nano set up can be 5-20 g..

I love my fw tanks..but I really like the sw set up. a challenge..yes..but only because I have so many tanks.
the other big expense in a sw set up..is the lighting..but if you run a FOWLR you don't need to go over board. with a reef set up you need lots of light..wel actually it depends n the coral andother inverts(anemones) as some will require a high amunt of light..but then agian..this isn't anymore expensive than a planted tank set up..