Fish slime and clouding eyes

Question
My son has a 135 gallon fish with several species in it that we are caring for him while he is off to college. The problem is two the water is clear and fish are ok.  Today they all have fish slime and clouding eyes and two of the older fish have died. I have medicated the tank with Meta-flex. All of fish seem to be going down quickly. Sucking for air, and we have done a water change each week.
Fish list; 1-paque(large), 3-tin foil barbs,1-tin foil barb(small), 1-green severn, 1-gold severn, 3-oscars, and 2-algea eaaters.  Two of the large tin foils have died.
Can you advise me my son is going to be real upset with me.

Answer
Good evening Stephen, thank you for your question. Wow!

Taking care of a 135 gallon fish tank is a considerable responsibility. Are you aware of the massive water changes a tank this size will need? With this stocking level, I would recommend weekly 30% water changes - that's 40 gallons!

Likely your problem is you are not changing enough water, and I wouldn't be surprised if you are overfeeding. The fish only need to be fed twice a day. Many of these fish are messy eaters. Only feed them enough so that all of the food is consumed in two minutes. You should not be seeing any food dropping to the bottom - a few pinches rather than shaking the can of flakes like a salt-shaker is recommended.

What kind of filtration is he using on his tank? You may not know for sure - that's ok. Let's hope your soon has adequate filtration in his tank. The most important part of filtration is keeping the filter media clean and not disrupting the biological filtration - the good bacteria that grows on the filter media and helps process waste.

Do the following. Discontinue the medication. All of this sounds like environmental disease that can be overcome with a step-up in maintenance. Believe me, I don't fault you! Unless your son is coming home for the weekends to service his tank, it is not advisable to leave a fish tank in the care of someone who is not previously experienced in keeping fish. Fish have very different requirements than dogs or cats. Everyone knows how to scoop out a litter box - how many people know that water changes need to be performed using dechlorinator, to remove sanitizer from tap water?

Okay, sorry! I just don't want you to beat yourself up. Unless your son left you with reading material and detailed instructions, what happened was a mistake you couldn't really help unless you took it upon yourself to take a crash course in fishkeeping. Here's what to do for now:

Do a large water change - 50% sounds right, 67 gallons. You'll need to add dechlorinator - Prime, for example, calls for 3 drops per gallon of water.

Vacuum the gravel. Here's more on how to do that:
http://www.firsttankguide.net/siphon.php

Service the filter. Depending on what kind it is (probably a canister for this sized system?) you will have different ways to go about accessing the media. The idea is, if he is using carbon, replace it. Hopefully he left you plenty of replacement media. If not, go to the local fish store and ask them to give you two bags of Chemi-Pure, by Boyd Enterprises. This is powerful stuff, it will really help absorb the wastes in the tank. If he is using biological media, like ceramic rings or plastic bio-balls - don't rinse these in hot tap water! It will disturb the beneficial bacteria. Use water from the aquarium's water change. Cold tap water might be all right, but preferably use tank water.

Perform these steps, and I strongly believe your tank will pull through. Discontinue the Melafix (is that what you meant?) since it's just too easy to overdose on medications, and this tea tree oil derivative is of questionable value in these situations. I hope everything gets better!

Good luck, feel free to write if you need any more help.
Nicole