https://ja.wikipedia.org/wiki/ヒシ科
ヒシ科 (ヒシか、Trapaceae) は双子葉植物の科で、すべて一年生の水草。現生種はヒシ属の数種のみからなり、ユーラシアと北アフリカに分布する。日本にはヒシ、オニビシとヒメビシがある。
https://ja.wikipedia.org/wiki/ヒシ
ヒシ（菱 、学名：Trapa japonica）はミソハギ科(クロンキスト体系ではヒシ科)の一年草の水草。
菱の実	3935	67.3%
ひしの実	676	11.6%
ヒシの実	1236	21.1%

Having read through the discussion below, I can't see a reason for dropping "Trapa japonica" from the gloss. Although it may be used more generally, ヒシ is the name of a specific species. 
Doesn't appear to be uk.

@@ -9,0 +10,4 @@
+<r_ele>
+<reb>ヒシ</reb>
+<re_nokanji/>
+</r_ele>
@@ -12,2 +16 @@
-<misc>&uk;</misc>
-<gloss>water caltrop</gloss>
+<gloss>water caltrop (esp. Trapa japonica)</gloss>

that's not how we gloss entries.  if it's also known as something, it gets its own gloss

@@ -13 +13,2 @@
-<gloss>water caltrop, also known as water chestnut</gloss>
+<gloss>water caltrop</gloss>
+<gloss>water chestnut</gloss>

After looking into it a tiny bit further (specifically, noting that one of the external links on the Wikipedia article is "Species Profile- Water Chestnut (Trapa natans), National Invasive Species Information Center, United States National Agricultural Library."), it would seem you're probably right ...or, at least, that the sources certainly don't rule it out.
Either way, I have to concede/agree it's best to just let the chestnut gloss stay.
(the chestnut term is best avoided, generally ...but should, indeed, be included here)

That said, it's probably best to avoid implying that water caltrop and chestnut are different things.

@@ -13,2 +13 @@
-<gloss>water caltrop</gloss>
-<gloss>water chestnut</gloss>
+<gloss>water caltrop, also known as water chestnut</gloss>

All 4 of my JEs (GG5, 中辞典, ルミナス, Grand Concise) gloss this as "water chestnut". The English 漢和字典 such as Nelson, etc. say 菱 means "water chestnut". The EJs I check all have 菱/ひし as the gloss of "water chestnut". I know the Wiktionary entry for "water chestnut" says: "The usage of water chestnut for the water caltrop is best avoided, so as to prevent confusion", but that's to avoid confusion with the Chinese water chestnut (Eleocharis dulcis), which is not an issue here.
I don't see how it's a misuse of the term, and I think that gloss can stay.

@@ -13,0 +14 @@
+<gloss>water chestnut</gloss>