'밑'은 물체의 아래나 아래쪽을 뜻하고, '아래'는 어떤 기준보다 낮은 위치를 뜻합니다. 첫 번째 문장에서 ‘바다 아래’라고 쓰게 되면 ‘바다’를 ‘기준’으로 하여 바다보다 낮은 위치에 물고기가 산다는 것이 되어, 물고기들이 바다가 아닌 그 바다보다 낮은 위치 어딘가에 산다는 뜻이 될 수 있으므로, ‘바다’라는 물체의 아래나 아래쪽에 물고기가 산다는 뜻을 나타낼 수 있는 ‘바다 밑’을 쓰는 것이 알맞다고 봅니다.
밑에 is often used interhangeably with 아래, but 밑 without the "에" means "bottom / lowest extremity" while 아래 means "under." The above Korean explanation points out that 아래 is not used to say "fish live under the ocean" as that would mean someplace below the bottom of the ocean.
When ‘ㄱ’ is followed by ‘ㅁ’ the ㄱ pronunciation changes to ‘ㅇ’. Here the final consonant in the word 식탁 is ㄱand the beginning consonant of the next word, 밑 begins with ㅁ so ㄱ’s pronunciation changes to ㅇ.
Source: Korean a Comprehensive Grammar by Jaehoon Yeon and Lucien Brown; Routledge, Taylor and Francis Group, London and New York, Published 2011.
Yeon and Brown identify this pronunciation changes as Nasal Assimilation and my comments have been made based on their text as provided on page 24 and page 25 of the reference source.
• Nasal Assimilation is a phonological change that occurs when a sound assimilates to become similar to neighboring sound.
• Further nasal assimilation happens when a stop consonant occurs before a nasal consonant. With respect to Korean, Yeon and Brown identify stop consonants as ㄱ, ㄷ, and ㅂ. This includes their counterparts: ㄱ, ㅋ,ㄲ; ㄷ, ㅌ, ㄸ; and ㅂ, ㅃ, ㅍ. They also classify nasal consonants as identify ㄴ, ㅁ, and ㅇ.
ㄱ, ㅋ, and ㄲ when followed by a nasal consonant its pronunciation changes to ‘ㅇ-ng’. ㄷ, ㅌ, ㄸ when followed by a nasal consonant its pronunciation changes to ‘ㄴ’. ㅂ, ㅍ, ㅃ when followed by a nasal consonant its pronunciation changes to ‘ㅁ’.
Here are four example words provided by Yeon and Brown.
1. 국물 pronounced 궁물 (broth). 2. 먹는다 pronounced 멍는다 (eat). 3. 부억만pronounced 부엉만 (kitchen only). 4. 묶는다 pronounced 뭉는다 (tie).
Duolingo tips: "래 and alx can both be translate to 'under', they are fairly different in meaning. 아래 refers to the whole space lower than a standard point which be anything (e.g. sunshine, moonlight, the sky, the sea level, etc.), but 밑 means either 아래 or the bottom of a tangible object. 자동차 밑 can mean either 'under the car'...or the bottom of the car." Duolingo tips also provided four sentence examples keying on this difference.