qstor
Adventurer
A version of the giant catfish electric is in the ToH 3 and a regular version is in Gaming Frontiers Magazine #2. I saw it was listed as not being done ie the version in the 1e MM II.
So here's a go at it
For your critique 
Catfish (Giant)
Huge Animal
Hit Dice: 7d8+21 (55 hp)
Initiative: +1 (Dex)
Speed: Swim 30 ft
AC: 13 (-2 size, +1 Dex, +4 natural), touch 9, flat footed 14
Base Attack/Grapple: +5/+21
Attacks: Bite +13 melee or slam +8
Damage: Bite 2d6+8 or slam 1d6+4 and poison
Face/Reach: 15 ft by 15 ft/5 ft
Special Attacks: Improved grab, swallow whole, poison
Special Qualities: low-light vision
Saves: Fort +8, Ref +3, Will +3
Abilities: Str 26, Dex 12, Con 17, Int 1, Wis 12, Cha 2
Skills: Listen +8, Spot +8, Swim +16
Feats: Alterness, Endurance, Toughness
Climate/Terrain: Temperate aquatic
Organization: Solitary or school (4-8)
Challenge Rating: 4
Treasure: None
Alignment: Always neutral
Advancement: 8-14 HD (Huge)
Improved Grab (Ex): To use this ability, a giant catfish must hit a foe with its bite attack. If it gets a hold, it tears the flesh.
Poison (Ex)
A giant catfish has poisonous feelers that deals initial and secondary damage of 1d6 Dex. The save DC is 14. The save DCs are Constitution-based.
Swallow Whole (Ex)
A giant catfish can try to swallow a grabbed opponent of a smaller size than
itself by making a successful grapple check. Once inside, the
opponent takes 2d6+8 points of crushing damage plus 8 points
of acid damage per round from the worm’s gizzard.
A swallowed creature can cut its way out by using a
light slashing or piercing weapon to deal 25 points of damage to the gizzard (AC 12). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.
Mike
So here's a go at it


Catfish (Giant)
Huge Animal
Hit Dice: 7d8+21 (55 hp)
Initiative: +1 (Dex)
Speed: Swim 30 ft
AC: 13 (-2 size, +1 Dex, +4 natural), touch 9, flat footed 14
Base Attack/Grapple: +5/+21
Attacks: Bite +13 melee or slam +8
Damage: Bite 2d6+8 or slam 1d6+4 and poison
Face/Reach: 15 ft by 15 ft/5 ft
Special Attacks: Improved grab, swallow whole, poison
Special Qualities: low-light vision
Saves: Fort +8, Ref +3, Will +3
Abilities: Str 26, Dex 12, Con 17, Int 1, Wis 12, Cha 2
Skills: Listen +8, Spot +8, Swim +16
Feats: Alterness, Endurance, Toughness
Climate/Terrain: Temperate aquatic
Organization: Solitary or school (4-8)
Challenge Rating: 4
Treasure: None
Alignment: Always neutral
Advancement: 8-14 HD (Huge)
Improved Grab (Ex): To use this ability, a giant catfish must hit a foe with its bite attack. If it gets a hold, it tears the flesh.
Poison (Ex)
A giant catfish has poisonous feelers that deals initial and secondary damage of 1d6 Dex. The save DC is 14. The save DCs are Constitution-based.
Swallow Whole (Ex)
A giant catfish can try to swallow a grabbed opponent of a smaller size than
itself by making a successful grapple check. Once inside, the
opponent takes 2d6+8 points of crushing damage plus 8 points
of acid damage per round from the worm’s gizzard.
A swallowed creature can cut its way out by using a
light slashing or piercing weapon to deal 25 points of damage to the gizzard (AC 12). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.
Mike