55 gal freshwater tank - critical water problem

Question
Hope you can help!

55 gal freshwater community tank, no live plants. Penguin 350 bio-wheel, air curtain and two airstones.
Cycled fine initially with 6 Rosies, added new fish slowly and no problems initially, but now having critical water problems.  

I suspect problems arose due to death of a pleco that went undiscovered for several days (I didn't suspect until the water went crazy, because it was rarely seen anyway) and changing filters in Penguin 350 I fear may have worsened my problem.  

Inhabitants:
1 yellow lab cichlid - 1.5"
1 blue kenyi - 1.5"
2 gold gourami - 3"
2 blue gourami - 3"
2 bala sharks - 1 - 2.5" and 1 - 3.5" (these I will be setting up a 100 gal tank for when they start to outgrow the 55 gal)
4 Rosies - 2"  (3 reg, 1 longfin)
2 silver dollars - about 2" diameter
2 plecos - 2" (will house with the balas when they outgrow this tank)

As I said, initially my tank cycled fine.  I added new fish slowly and got small spikes, but then back to normal each time.  About 4 weeks ago I lost a pleco, unbeknownst to me until the ammonia went crazy.  I found and removed the dead pleco, did a thorough gravel vac and changed the filter cartridges on my Penguin 350.  Ammonia initially dropped, but then about 2 weeks ago, along with the nitrites and nitrates, spiked very high and my Ph continues to drop, even with water changes.  It's now down to about 6.4.  I have heard that that too low a Ph can inhibit the bacteria that consume the nitrites.  Therefore, was going to buffer the water with baking soda and see if that stabilized the Ph a bit, but hestitant to do anything to raise the Ph due to the high ammonia readings right now.  

I'm continuing to do approximately 15-20% water changes every other day and vacuum about 1/2 the gravel weekly, alternating sides, but the toxins continue to remain high and the Ph, while it increases to about 6.8 or so initially when water is changed, within about 24 hours drops again to the 6.4 or 6.6 range.  Tap water Ph is about 7.5.

Right now my levels are as follows:

Ammonia - 6 (API liquid reagent test)
Nitrites - higher than my Mardel test strip reads
Nitrates - bounce between 60-80...lower with water changes and then back up within about 24 hours
Alkalinity on the very low side of normal
Hardness lowish side of normal.
Temp - 76

I'm considering adding Bio-Spira with the next water change to see if that would help get it under control, but wondering if you have any other suggestions.  I have no other tank right now to move the fish to (outside of a very small quarantine tank) and "start from scratch" and re-cycle it.

I have been adding aquarium salt with the water changes, to reduce the nitrite uptake and protect the fish. Cut feeding way back.  Lowered the temp a bit (had been keeping it around 80) to help reduce ammonia toxicity.  All the fish continue to do well...ie acting normally, eating well, etc.  But I know if I don't get this water under control soon, that won't last long.

Can you please help?  Any suggestions are welcome.

Thanks!  

Answer
Hello,

First off, I recommend changing all your test kits from strips to the liquid or drop kinds. I have seen far too many inaccurate test strips in  my life to rely on them any longer.

Second, changing the water every few days is good, but now that your gravel is syphoned I recommend leaving it for a week before syphoning again. This is simply because beneficial bacteria need an opputunity to grow in your gravel and establish themselves so that they can take over waste control.

Third, unfortunately changing your filter cartridge in a time of peril probably set your beneficial bacteria count WAY back, so this problem will definitly take some time to clear up.

Fourth, there is a product called Cycle which speeds up the nitrogen cycle by adding beneficial bacteria to your water. I highly recommend this product.

Fifth, which such a large tank like yours that is so full of fish, it will take a while to see a significant improvement. Don't worry about ph right now, once everything else is solved then you can concern yourself with that (and don't use baking soda!) You may find that if you were using test strips to check the pH and switch to liquid tests, there won't actually be a problem.

I hope this helps.

Regards,
Stephanie