Mystery Death

Question
QUESTION: Hi Michael,
 I own a 30 gal tropical fish tank. I have 3 irridescent sharks, one gold fish, 3 sword tails, 2 mollies, 1 platty ,1 betta fish, 1 bala shark and one male guppy.

Goldfish is not a tropical fish but i realised that its a cold water only after i set up my tank. Its been 4 months i set it up and my ammonia, nitrites is 0 and nitrates <25PPM. I change 25% water every 3-4 days .. in a way its 50% water changes in a week.
I have an emperor 280 filter and an aqua clear filter for a 50 running, with 2 air diffusers and an 8 inch air curtain.

i noticed one of my male guppy had swollen tummy and eyesy bulging .. so i am treating him with maracyn two for dropsy and the eye symtomn(forgot the name of the disease).
the same thing happened to my platty .. he never made it

Two days ago one of my balashark had died all of a sudden.

Do u think my tank is over crowded ?

I feed them once a day. I feed the goldfish pro-gold and the other fish a variety of tropical fish food what they would eat in 3 mins, there is never any left over food.

I am not sure how to get over this kind of deaths, i also put some aquarium salt.

I feed them thawed peas with their shells removed once a week.
Pls let me know where i am going wrong.

Thanks a lot,
vamsi

ANSWER: Vamsi,
you may be doing too much water changing and not letting the tank settle in correctly.
do 25% a week and thats more than enough.
Do you know what the PH is?
Temp?
hardness?
if those are in line then its time to look at the fish themselves.
There could be something going on thats hard to see.
Check the gills closely for parasites.
Even with the GoldFish he will be fine at 78 degrees.
Let me know what you find.


---------- FOLLOW-UP ----------

QUESTION: Thank you for your reply . Here are the other water parameters  
Ph 6.8
Temp: 78F maintained by a heater.
hardness i am not sure .. how can we measure this? I use well water. Whats the acceptable range for hardness?

Prior to this i treated all my fish with Jungle parasite clear.
I will switch to 25% water changes from now. Is there any other precautions or something else which i am missing?

Answer
hmmm,
temp good,
ph good
is it difficult to get your water to soap up?
if not, then your water is ok, if it takes alot then its hard, if it takes only a drop or two then its soft. rough parameters but close enough.
Just out of curiosity, next time you buy fish get them somewhere else and see how they do.
At this point it seems you are doing what you can do and doing the correct things.
Unless we get some new symptoms we are in a "wait and see" situation.
Keep me informed.