What
@talking.domains said is pretty much the essence of the difference between singular and plural in the majority of cases. However .. there most certainly are plenty of exceptions, so NEVER assume what works for one domain will work for another. Your specific word/term might or might not be good in singular, plural, both or neither .. it really depends on the specific word(s).
As mentioned, the plural is usually (NOT always) better for selling the specific word:
- Fish Tanks, T-Shirts, Books, Movies, etc)
But singular words/terms are what usually make for the most in demand GENERIC brands .. meaning that the word can be repurposed to be anything:
- Apple, Uber, Canon, Puma, etc
While you certainly could sell apples at apple.com, most people would tend to go to apples.com when looking to buy. At the same time, apples.com wouldn't really be a place people would ever feel comfortable going to for computers.
FishTank is actually the perfect typical example, as indeed as TD mentioned, you would instinctively go to FishTanks.com when looking to buy a fish tank ... while FishTank.com would make for a great online dating brand (FishTanks.com would not work at all in that way).
Again though ..
NEVER base your decision on what we've said ..
each domain and term is unique .. do NOT assume your word/term will work in the same way .. the only way to truly get good/accurate advice is to say the exact term .. although obviously that's not really a good idea if you haven't bought the domain yet.