This is my first post on the forum, hope I put this in the right place!
I have had a couple of Goldfish in a 30L tank for a while now and recently went to the fish store to get an Apple snail to help clean up the tank

After sitting him in the tank for a while in his bag I set him free and he went down and burried himself in the gravel as I thought he would do at first, after a while he came out to explore his new environment. The Goldfish also came to check him out and gave him a nudge or two, he just shrugged it off.
Now my worry is this, the little guy doesnt seem to actively compete for food. I bought some frozen mosquito larvae and some sinking catfish/loach granules when i bought him that the fish shop recommended. The problem is that whenever i drop some of it in the water the goldfish polish it all off before the little guy gets a chance! even if they are already eating their flakes.
Now I have seen him eat a little, but only a few granules and am a bit worried he is not getting enough food. I have only had him a couple of days and he seems fine and is quite active, infact i have only seen him bury himself once more, he seems happy sitting about under the little bridge

I read that they are more active for seeking food at night and so popped a bit more in in the evening, but again the goldfish gobble everything up before he even moves from his spot. He does then seem to come and "sniff" around the area for leftovers, but its a bit late.
What am I doing wrong? I don't want to put too much food in there as won't the goldfish keep on eating till they pop? Or am I underfeeding them too?
Any advice appreciated, sorry for the long post!