There are seven general exceptions in the 2nd declension. Instead of ending in -is, seven nouns end in -s ****.
Essentially, the nominative singular is all that is different. According to my grammar book, the genitive singular is also -s instead of -a with a consonant palatization change, but every time I go by the book when I translate into Latvian, my husband corrects me.
So, apparently, it depends on who you talk to and what they think sounds good. I don't think it's wrong either way and you'll be understood regardless of which you choose.
The seven nouns are:
- akmens : stone
- asmens : blade
- ūdens : water
- rudens : fall, autumn
- zibens : lightning
- mēness : moon
- suns : dog
Then there's sals (salt). It's technically still a 6th declension noun but it's moving into the 2nd declension where it'll be an exception…. and no one really knows which one it's actually in.