Stringy white feces is a great indication of a digestive issue but it does not always mean internal parasites, what your actually seeing is the lining of the intestinal track. This irritation can be caused by improper diet, low quality foods, and ofcourse internal parasites. So the treatment is not a one size fits all I'm afraid.
If at anytime you start to see other symptoms such as refusing to eat or mouthing food, reclusive behavior, or acting lethargic then treatment may be needed but for now just keep doing what your doing.
I'm not a fan of your food choice.
The key is high quality easily digestable proteins. So when your looking for foods it's not about the protein percentage, but rather the quality of the proteins themselves. Food that derives it's protein from high soybean, corn, wheat flour and other fillers is not nearly as digestible as a high quality pellet that utilizes the entire fish not just the left over bi-products from other industries; such as Whole Antarctic Krill Meal, Whole Herring Meal, Whole Squid Meal is what your looking for. Coincidentally that's the same stuff New Life Spectrum uses, I'd feed that exclusively and call it a day.
Here is list of Dianchi ingredients: Not a very impressive list:
White fish meal, wheat germ, wheat flour, krill, spirulina, soybean meal, brewer’s yeast, calcium montmorillonite clay, powdered milk, vitamin mix, mineral mix, garlic, protease and natural gum.
One of the biggest differences in fish foods is where the main protein ingredients derive from. Is it fish meal or whole fish meal, fish meal consists primarily and potentially exclusively of scales, skins, and bones where as whole meal utilizes the entire fish not simply the left over bi-products from other industries; remember what old old saying about what hotdogs were made from(lips and butts) same applies here. What you looking for is quality highly digestible ingredients such as Whole Antarctic krill meal.
The most important fact to remember here is that fish digestive systems are designed to handle certain proteins. Freshwater/Marine proteins and fats are very different than their mammal counterparts. Their digestive systems are well designed so that if the food is of a protein that can not be digested properly it will simply be expell this excess immediately. The primarily function of breaking down food is a provide energy, that which can't be processed generates a lot of waste since they're expelling what can't be used to benefit the fishes body. Proteins are made of amino acids which contain nitrogen, carbon, hydrogen, and oxygen, so as the fish is breaking these non-native foods down they in turn make a lot more nitrogenous waste which as everyone knows will effect water quality.
The last couple of points I will make are:
Another large issue comes into play with regards to fillers and binding agents. You'll even see some lower end products which use multiple types of wheat, the ingredient list follows a practice known as product splitting, with three different wheat products listed. When combined, wheat may very well be the primary ingredient. Which is by no means the building block of a quality product.
The last point I'll make is the ash content, ash comes from bones and shells, the percentage of ash at 10% or higher usually indicates left over white fish, shrimp or loster heads are used rather than the entire fish again. Since the fish food market is self regulated you'll even see some brands leave the ash content off since they're not bound by any laws; if you see this its not fit to feed any living creature.