This is a brief mention of a characteristic of items in general in Island of Happiness. Any 'perishable' item has a Rank, Quality, Size and Freshness quotient. As time passes, the Freshness diminishes first, but it gradually will affect the Quality and Rank as well. Unless you have built a Refrigerator and Cabinet from Mythic Stone or have a 'Quality Improver' Maker to 'refresh' your items, they gradually will diminish in Freshness, Rank and Quality.
This is the case not only with respect to Crops and Fish but with respect to Wonderfuls and Suns. They actually are perishable items. The Suns purchased from Chen's Market will be of random Rank, Quality, Size and Freshness. The Wonderfuls purchased from Chen's Market or obtained on the 255th Floor of the Mine likewise will be of random Rank, Quality, Size and Freshness. It is only when you obtain a Wonderful as a prize for winning a Contest that it will be of S Rank, maximum Quality, Maximum Size and Maximum Freshness.
The reason I mention this is because any Wonderful or Sun that is 'used' by placing it in a Tool or Greenhouse respectively will diminish gradually. Furthermore, Wonderfuls 'stack' differently from all other items when set in a Tool. Ordinarily, items of the same variety will not stack automatically unless every characteristic is identical. In other words, they go into the same slot automatically only if Rank, Quality, Size and Freshness are identical.
When Charlie sets any Wonderful in a Tool, however, he will stack all Wonderfuls of the same colour. That means that an S Rank Yellow Wonderful, for example, will be stacked with a C Rank Yellow Wonderful, bringing its Rank, Quality, Size and Freshness DOWN immediately. Of course, using the 'half full/half empty' perception, you could say that an S Rank Wonderful would 'refresh' the C Rank Wonderful, boosting its Rank, Quality, Size and Freshness to some extent.
In any case, you can boost the Quality of any item by throwing it into a Quality Improver Maker, but that is the most expensive Maker in the game and you will be unable to afford one until you have made significant progress.
Remember though that the actual potency of any Sun placed in your Greenhouse will determine the amount of 'Sunlight' that your Crops receive daily. Try to use the best Suns in your Greenhouse, replacing those that have lost their potency. Toss any used Sun in the shipping container to complete your shipping list.