In my experience there is more difference between seedlings individually or from cross to cross than there is between hardies and tropicals, day and night bloomers etc. I had a seedling in my greenhouse pool that just sprouted and grew in a pot. I went back and forth in my mind as it grew, is it a night bloomer is it hardy is it a day bloomer. As it got bigger I decided that it was a hardy but soon it became apparent that it was a night bloomer.
At that point I knew I couldn't tell so I just grow them out to find out.
If all you have in that pool is yellow queen and tropicals then the chances that they are trops is very high.
I have Yellow Queen and I might remember seeing a seed or two but my memory tells me that it makes VERY few.
Tropicals are huge seeders and they sprout in pots and along shore in my ponds. Most tropical seedlings that I see are light green. White night bloomers are usually light green and other night bloomers tend to be brownish with arrowhead shaped submerged leaves. If your seedlings are from your Yellow Queen then the majority of them will have multy colored and or marbled submerged leaves that are arrowhead shaped. The majority of day blooming tropical submerged leaves will be light green and rounder more like surface leaf shaped or wide arrowhead.
Submerged leaves can be like a very thin blade of grass or wider like a wide blade of grass with no sinus. They can be like a wide arrowhead or a very long thin concave sided arrowhead with long barbs to a triangle. They can be round and flat with an overlapping sinus just like a surface leaf or crinkly like a piece of lettuce. They can and will usaly change shape and color as the plant gets bigger. Usualy from thiner to wider and lighter to darker or more colorful as it grows. They can be translucent or almost black or red as blood or purple or shine with the color of gold. They can be multy colored like a Coleus leaf. Most of this kind of variation will be seen with hardies and inter sub-generic crosses between hardies and tropicals. Most tropical seedlings are pretty bland by comparison. More uniform in shape and color. But I have very little experience with tropical seedlings because I don't deliberately breed them I just grow the volunteers that pop up everywhere.
Does your Yellow Queen flower well for you? Mine almost never flowers. I'm thinking of getting rid of it.
Here are some various seedling pics.
