Cloudy tank and lethargic fish?

I have two gold fish. One is a red fan tail who is about three years old and her body is about two inches with very long beautiful flowing tails. The other is a black Moore who is also around three years old with a bit bigger body and shorter fins. I have had both since they were very tiny. They live together in a 20 gallon tank and have hopes to increase their tank size but right now don't have the space or money. They have been very healthy besides a rough start for my fan tail. When I first got her from wal mart I knew nothing about the proper care of good fish. She suffered for ny ignorance but as soon as i saw her in distress i did everything i could to help her and ended up falling in love with her, the hobby of keeping fish and i learned allot. She has been very healthy since. I added the black Moore once my fan tail was strong and doing well. They have lived together for two years and have been good buddies. By the way i have just guessed their es i really have no idea. Anyway for the last 6 months i have noticed the tank getting cloudy very quickly. I do 25% water changes weekly and 50% monthly. The tank has a whisper filter that is for a 15-20 gallon tank. I feed them high quality slow sinking pellets and soak the pellets in fresh garlic juice once a week. They also get thawed frozen blood worms and fresh fruits and veggies on a regular basis. My other concern is my fan tail has been very lethargic. I don't see her swimming around investigating or playing with her buddy like she use too. The water tests out good and the black Moore is very active. I am wondering if I need to add another filter until I am able to get them a bigger tank or if there is anything else that could be the problem. I have grown very attached to my pets and I want to do everything I can to ensure they are healthy and happy, but I know I am still a novice aquarium keeper and have allot to learn. If you have any suggestions please share :)