Brackish Eel

Most FW eels and many eel-like fish will get to big for a 25. There aren't any morays that will thrive in FW or even mild BW for thier entire lives, especially is such a small tank. The FW American eel will quickly outgrow the tank, as will many of the so-called FW/BW morays--snowflakes are often found in FW/BW estuaries as juveniles. These migrate to marine waters as they age.

A single spiny eel, or a peacock eel, would be okay for a while, but I wouldn't suggest adding much else to the tank--maybe some top-level fish like hatchets. Either way, a well covered tank will be a must--they are escape artists.

Wups--forgot to add. The easiest plants to care for will be those that do not get planted in substrate, but rather attach themselves to rocks and wood. Anubias, java ferns, and java moss are all attractive low light plants that I've had very good luck with. Crypts--planted in the substrate--are very hardy under stable conditions, but tend to melt if there are sudden or frequent changes. All of these plants can be adapted to brackish conditions, though java fern has done best for me.

We had two Snowflakes at the Pet Store once...they got white spots that was treated with Marcyn but eventually they both died before being sold, (the white stuff came back.) It wasn't until after they were sold that it occured to use they may have been saltwater or brakish.

Check out this info on eels: http://www.predatoryfish.net/ibforums/index.php?act=ST&f=25&t=38373

Gymnothorax buroensis
Vagrant moray

Gymnothorax buroensis (Bleeker, 1857)
Family: Muraenidae (Moray eels), subfamily: Muraeninae

picture (Gybur_u5.jpg) by Randall, J.E.

Map
Order: Anguilliformes
Class: Actinopterygii (ray-finned fishes)
FishBase name: Vagrant moray
Max. size: 35.0 cm TL (male/unsexed; Ref. 2334)
Environment: reef-associated; brackish; marine ; depth range 0 - 25 m
Climate: tropical
Importance: fisheries: of no interest
Distribution:
Gazetteer Indo-Pacific: Red Sea and East Africa (Ref. 33390) to the Tuamotos, north to the Ryukyu and Hawaiian islands; throughout Micronesia. Eastern Central Pacific: Costa Rica and Panama (Ref. 9324) and the Galapagos (Ref. 2334).
Diagnosis: Dorsal spines (total): 0-0; Dorsal soft rays (total): 0-0; Anal spines: 0-0; Anal soft rays: 0-0. Dark brown with irregular dark spots anteriorly, interspersed with lighter granulations that form diffuse crossbars on the tail.
Biology: Occurs in shallow lagoon and seaward reefs, primarily found in the surge zone (Ref. 9710). Lives between rocks and corals.
Red List Status: Not in IUCN Red List , (Ref. 36508)
Dangerous: harmless

Gymnothorax favagineus
Laced moray

Gymnothorax favagineus Bloch & Schneider, 1801
Family: Muraenidae (Moray eels), subfamily: Muraeninae

picture (Gyfav_u5.jpg) by Field, R.
Order: Anguilliformes
Class: Actinopterygii (ray-finned fishes)
FishBase name: Laced moray
Max. size: 300 cm TL (male/unsexed; Ref. 9710)
Environment: reef-associated; brackish; marine ; depth range 1 - 45 m
Climate: tropical
Importance: aquarium: show aquarium
Distribution:
Gazetteer Indo-Pacific: Red Sea (Ref. 33390) and East Africa to Papua New Giunea (Ref. 9710), north to southern Japan, south to Australia.
Diagnosis: Dorsal spines (total): 0-0; Dorsal soft rays (total): 0-0; Anal spines: 0-0; Anal soft rays: 0-0; Vertebrae : 140-143. Basically white with black blotches and interspaces forming a honeycomb pattern (Ref. 30404). Blotches variable between individuals and size, often in relation to habitat - those in clear coral reefs usually have proportionally less black than those found in turbid waters (Ref. 30404).
Biology: Inhabits reef flats and outer reef slopes of continental reefs (Ref. 9710). One of the two largest of Indo-Pacific morays. Feeds on cephalopods and small fishes (Ref. 30573). Large adults may be aggressive (Ref. 9710). Minimum depth reported taken from Ref. 30573.
Red List Status: Not in IUCN Red List , (Ref. 36508)
Dangerous: reports of ciguatera poisoning

Gymnothorax fimbriatus
Fimbriated moray

Gymnothorax fimbriatus (Bennett, 1832)
Family: Muraenidae (Moray eels), subfamily: Muraeninae

picture (Gyfim_u7.jpg) by Carvalho Filho, A.

Map
Order: Anguilliformes
Class: Actinopterygii (ray-finned fishes)
FishBase name: Fimbriated moray
Max. size: 80.0 cm TL (male/unsexed; Ref. 1602)
Environment: reef-associated; brackish; marine ; depth range - 45 m
Climate: tropical
Importance: fisheries: subsistence fisheries
Distribution:
Gazetteer Indo-Pacific: Madagascar (Ref. 33390) to the Society Is., north to southern Japan (Ref. 559), south to Queensland, Australia; throughout Micronesia.
Diagnosis: Adults pale with small black spots on face; black spots proportionally much larger in young (Ref. 30404).
Biology: Occurs in lagoons, reef flats and seaward reefs (Ref. 1602). Prefers protected inshore waters among dead corals, common in harbours and small caves (Ref. 30404). Probably nocturnal (Ref. 1602). Feeds on fishes and crustaceans (Ref. 9710).
Red List Status: Not in IUCN Red List , (Ref. 36508)
Dangerous: harmless

NewbieForever, if you really want an eel but can't get a bigger tank at the moment you can probably keep a "freshwater" moray (Echidna rhodochilus) in the 25 gal for some time, but definitely plan on getting a bigger tank for it eventually (at least 55 gal or something like that).
The best eel for a 25 gal would probably be a spiny eel of some sort, although I have no experience on those.

i picked up a 5" peacock eel about 3 weeks ago.

its in a sand substrate 20g long tank and eats frozen blood worm and frozen mysis shrimp. its little face is so pointy i dont see how it could open its mouth enuff to eat anything much bigger.

he has plenty of opportunity because the tanks also has 2 small gobys, a bunch of ghost shrimp and some tiny guppy fry. he stays buried unless its feeding time. then he's out and about.

i am glad he's not brackish as brackish eels tend to want full salt water later in life dont they?

I had some siney eels before until my puffer came along and nipped off the nose.
They were in slight BW 1.005 did fine.