maintain tank water in good condition

Question
Hi Stephanie
A) Today I saw two of my fishes having white spots again.  This is their third time white spots coming again.  About four months ago, it was their first time to be treated by 'Mardel' for 4 weeks.  One week later, white spots came back.  Then I started their second time 'Mardel' again for another month.  After ten days (today), two of my fishes have white spots again.  I went to enquire the fish keeper who recommended me to change another stronger antifungal 'Pimafix'.  I am going to use that medication for 7 days as a course.
1) What wrong with my fish tank?
2) or my fish immue system has problem.


B) My another 10 gallon fish tank has 1 paradise gourami and six head and tail tetras, hang out filtration, some real plant, drift wood, fluorescent light.  Today the paradise gourami's color was so strange.  The end of its half part of body turned darken but the another half part of its body remain the normal color. Look like suntanning.   Any internal infection?  Should I destroy it?   
Ming

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The text above is a follow-up to ...

-----Question-----
I will have my 3 weeks away.  I can only assign my daughter to take care of my fish tank.  My fishtank is 55 gallons.  My daughter is 14 years.  I believe that she can help some simple water change weekly.  eg.  rinse cartridge weekly, take the water out of tank by using small bucket and replace fresh water with conditioner into the tank.  In order to minimize  ammonia trigger out of fish drops, the fishes are going to be fed twice a week.   All fishes in my tank have more than one year old.  Are they ok to be fed in such small quantities?  Will the tank water be kept in good condition with small partial water change?  The fishes we have 2 angels, 5 rasboras, 3 golden tetras, 4 rainbow fishes, 1 black neon tetra, 1 cherry barb and 1 sesame algae eater.  Actually, my tank still have many room for them (my fishes only used my tank 30 gallons only.).
-----Answer-----
Hello,

Partial water changes will be a very good way to maintain the water quality while you are away. With these partial water changes, I would actually recommend feeding your fish more then twice a week. That may be too little for some of them, and could cause illness. Feeding them a small amount, once per day should not cause ammonia peaks and should be enough to keep them happy and healthy. Make sure you show your daughter how much to feed them so that she does not overfeed.

I hope this helps.

Regards,
Stephanie

Answer
Hello,

a) If your fish have white tufts on their body, then you should proceed with using Pimafix, but from the sounds of it your fish don't have a fungal infection at all, but rather a parasite. If it is little white dots all over their bodies, then you should treat with Maracide or another parasite medicine, as they likely have ick.

b) Sometimes fish become more or less colorful as time progresses and they get older, healthier or otherwise. You should observe your fish carefully. If he is eating and shows no other symptoms of disease then he should be fine. It may also be a good idea to test your water for nitrites and ammonia, because sometimes those two factors can cause color change in your fish. Usually though they cause loss of color, not darkening of color.

I hope this helps.

Regards,
Stephanie