Most people call them planarians but they are probably round worms. They inhabit every aquarium but often go unnoticed. Their population will explode if uneaten food is left laying around in the tank ect. Despite their tiny size, Gourami's love to eat them.
Sadly more and more places seem to be offering them for sale with little or no info, meaning that most are doomed. But it may not be one of those, I think some Pangasiidae are also referred to as Shark cats (but those are equally a bad choice).
Re: Potential benefits of anoxic/suboxic sediments in your tank
Heater cables in planted aquaria fulfill the same function by creating small convection currents through the substrate. It is such a shame that they have lost out in the 'battle of the methods' because from time to time the so called high tech method fails due to hydrogen sulphide forming in the substrate and it is the use of an heater cable which prevents it.
Just to make it clear for those who don't know, a u/g heater cable isn't used to heat the tank at all it is low powered device i.e 25W for a 50 gall tank. Its sole purpose is to create a very small flow through the substrate in the way described in the above post.
Sorry to say but this tank has been set up to fail. It was a very bad move to add any fish before the tank was cycled and the fish which have been stocked would never work together.
Shark cats - if they are Ariopsis seemanni only very young fish live in fresh water, they require ever increasing amounts of salt in their water as they mature until as (14 inch) adults they require full sea water.
Silver tip tetras are well known for being fin nippers and should never be kept with fish like guppies.
Pleco - depending on the species may grow far to big for your tank.
Botia of any kind should never be introduced to an immature aquarium or white spot is almost guaranteed, and to make matters worse they don't tolerate medication very well at all. It may also eat your shrimp once it is well.
Lots of problems to deal with here.
I would treat the tank using Waterlifes Protozin. With Protozin the dose is built up gradually and because of this it is slightly kinder to Botia although nothing is certain. One the white spot is cured you need to take all the fish back to the retailer and cycle your tank. Once it is fully cycled do a little research about the fish you intend to keep and make sure they are suitable for your tank and each other.
Looking at the photo in message 4 there is no way that this is an over stocking issue.
The fish even in the 'before' photos are in really poor condition, very small for their age, very underweight and just waiting for an opportunistic infection to strike, apart from the fin rot the fish appear to have a systemic bacterial infection. Taking all the available information I would guess the fish have been kept long term in poor water conditions possibly caused by:
Un-cycled tank. Insufficient water changes. (The water does appear to have a yellow tinge to it, possibly indicating high organics). Poor filter maintenance. (i.e washing the sponge in tap water). Poor diet.
Sorry to be harsh but those fish are beyond treatment. If they were mine I would have no hesitation in intervening otherwise they are going to suffer a lingering death.
Established fin rot infections are extremely difficult to treat and are usually beyond the scope of over the counter fish shop remedies. Even treating with antibiotic injections under the guidance of a vet can fail. I know of a particularly stubborn case on a high value koi which had the infected part of the tail surgically removed by a vet after all other methods failed to help. I'm telling you this to warn you not to expect to much from the various pet shop potions.
With regard to tank size it's simply a matter of common sense and opinion because there is little or no actual research in this area. There are many things to consider though. Allowing the fish enough space to behave naturally is something I use as a guide.
Any electrical goods not specifically intended for use in a wet/damp/humid situation are likely to fail if they aren't properly protected. Not only that, but it could prove quite dangerous too.
Having said that, I imagine there are lots of people who have done this or similar things and got away with it, but speaking personally (and as a qualified electrician) I wouldn't do it myself.
I have a an Exo Terra Terrarium with the official canopy for my frogs and due to the design there is a humidity problem and the lights were forever failing until I adapted the canopy to keep it perfectly dry. The lights (?17 each) were lasting 2 - 3 months and now last 2.5 years and counting.
For breeding tanks an air operated sponge filter is the only way to go. Other filters pose a serious threat to the fry, even if you use old tights or similar to cover the filter intake. The suction is still to powerful and the fry simply end up stuck to the fine mesh where they quickly perish. If you just clean the sponge so that there is a good water flow through it rather than make it spotless a biofilm will form and this offers the fry somewhere to graze from safely.