You made an understatement when you said that "the petstore completely misled us about how best to establish a new tank" .
Not only is the tank too small for the fish you have but these fish are not the appropriate types to use when cycling a tank. I don't to what extent you're aware of the cycling process but just in case I've listed a link below which will supply you with a brief overview.
As your tank is passing through the cycling process and hopefully nearing the end of it, the presence of nitrites is inevitable so what needs to be done is to try and not worsen the nitrite situation.
Ok as for trying to put nitrites under control.
Drastically reduce the amount of food you are feeding the fish. Feed the fish only a small pinch of food once every other day. Excess feeding and the subsequent fish waste produced are fuel for nitrite growth.
Normally I would suggest adding aquarium salt instead of sea salt to the water but as you have catfish in the tank it's best to not use any kind of salt as catfish do not react well to it so leave the salt out.
Do not change or clean the filter media as this is where the beneficial bacteria that breaks down nitrites will colonize. Once all traces of nitrites have been eliminated should you then change/clean the filter media.
Continue to do partial water changes every 3-4 days. Change between 15%-20% of the water when doing the water changes.
Try not to add too many additives to the water, i.e., bacteria, nitrite removers, etc.etc. as they will not be of any help and some might even take the tank back a few steps.
Please keep in mind that what I've recommended is not a quick cure as none exists.
What's happening in the water is normal for a fairly new tank set-up such as yours it's just a matter of trying to lessen the impact.
On a final note: be wary of advice given by pet stores/shops as many a time their number 1 priority is making money and not on a customer's healthy aquarium.
Basically problematic fish tanks and aquariums are good for pet store business.
Best wishes and please let me know if you have any questions.
Thanks so much for your response, and helpful information.
Many many thanks for your help -- your advice is much appreciated, as I'll be jumping into whatever action you advise once home from work!! :)
The only way to counteract/dilute salt in aquarium water is via partial water changes obviously not adding salt when doing them. Apparently the catfish have been able to survive the exposure to salt and it's good that you didn't overdose.
You're feeding properly--keep it up. No surprise that ammonia=-0-, the nitrites consumed all of it.
Yes--do not change any filter media for now even the mesh-like filters. As mentioned prior, the filter media is a vital area in terms of the formation of "good" bacteria.
The instructions which stated to replace filter media every two weeks are correct---but this would only apply to a cycled tank. Actually the same holds true for gravel/substrate as this is another important area where "good" bacteria colonize--not the time to change/remove while tank is cycling.
Wouldn't help to re-inset old filter media as the benefical bacteria has since died off.
Re: additives---the only thing you should be using is a product to dechlorinate tap water and make sure the water that you'll add to the tank is already dechlorinated before adding to the tank. Some fishkeepers add tap water to the tank and then add the dechlorinator---risky move! The bottled bacteria or supplement, honestly is a waste.
I believe the so-called bacteria contained inside these bottles is actually dead bacteria and as you have seen your tank hasn't cycled any faster despite using it.
As for the frequency of doing partial water changes when nitrites are high---it's kinda like a trade-off--the more frequent you do the water changes the longer it takes the water to cycle but it makes life a bit easier for the fish. I felt 3-4 days was a moderate/in-between course of action to take but if the nitrites are off the chart as you mentioned then yes it would be best to do more frequent partial water changes but remember as ironic as it may sound you need the nitrites in order for the tank to cycle so don't change too much water.
Great question you ask concerning available products to speed up the cycling process.
There is a product called Bio Spira which claims to instantly cycle a tank. Click on link below for a view of the product. The only thing I can tell you about it is that I tried it only once and it did not instantly cycle my tank although it lessened the amout of time it took the tank to cycle but to be fair this product needs to be refridgerated from the moment it is manufactured until the time it reaches your home and still needs to be refridgerated at home so if at some point the batch I used was out of refridgeration (while in transit from the various hands it passed through) it's possible it lost some of it's potency but obviously I can't say for sure if the product really doesn't do as claimed or I just got a defective batch.
I SO appreciate your help, and will follow this to a tee! Last itteration to clarify, I promise... just want to be sure and get it right, and you're the BEST help I've found yet!!
I am then assuming that:
Thanks for this final clarification - SOOO much appreciated!
Vacuuming the gravel when doing water changes is fine but don't overclean it as too much cleaning of the gravel can affect the amount of beneficial bacteria there is in the gravel/water.
When doing water changes vacuum only part of the gravel bed. During the next water change vacuum another part of the gravel bed, etc.,etc.
No need to vacuum more than when you do the water changes as remember you're only feeding the fish once every other day so there shouldn't be that much waste product.
For now while the tank is cycling it's best to not clean the ornaments. Once the tank has finished cycling then you can clean the ornaments and decor.
And please remember to be patient- I truly understand that this is a very frustrating time in the life of your aquarium but unfortunately we can't rush or change the biological processes that naturally occur.
I'm glad I've been able to assist you and I oh so understand that pet shop advice is sorely lacking. Lemme know if you have any more questions or inquiries.