Hi,
I'm afraid it's definitely not a good sign, I can't think of another reason for a fish not maintaining buoyancy than swim bladder problems.
Fish can live with swim bladder problems, that on its own wont kill them, but it can be distressing for the fish if it constantly struggling to right itself or if it is unable to eat. In situations like this only you as the carer can decide when the fish has no quality of life and whether to intervene in the most humane way possible.
Sorry if that's a little gloomy, but there is no cure for a failed swim bladder, don't give up on him just yet though, unless he is suffering, give him a chance to see if he regains his balance. I wouldn't recommend adding medication unless there's a definite need for it though, that won't help a fish on the edge.

Yes you can use your existing test kit, its only saltwater it won't work for.

Oh dear, am sorry to hear that. Most of us, even with experience, have made mistakes and lost fish though. I made a catastrophic boo boo with a heater when i had to move my well established marine tank to redecorate.. killed everything overnight!

I would work on getting a bigger tank for your two comets though, they will grow to about 12 inches in length, and they will love you for giving them swimming room especially as they grow. A 35l tank is much better suited to a small tropical set up, your little guys will be fine with good maintenance in the short term, but the sooner you can upgrade the better.. Realistically, in the long term you should probably consider a 6ft tank.. Or a pond if that's possible.

Hiya,
How is the shubunkin now, and where is it?

It is likely that your existing fish see the new fish as an intruder, or a potential mate, they can be pretty boisterous, and even sociable fish can be quite territorial in a smaller environment.

Hiya,
Don't worry too much, he's only been with you a week, and its possible he's only ever seen flake food until he came to live with you... Anything that is alive (brine shrimp, daphnia and bloodworm if u can find them) will attract his attention.
They will pick up food from the bottom of the tank, I have kept a few bettas over the years, all have learnt quite quickly what is food and to find it wherever it is... Although It is possible that yours may not. It may help to not feed him for a day before you try new foods, I am not suggesting you starve him, just he will have no inclination to try new food if he is not hungry.. A day without food once a week is not a bad thing to make routine anyway!
Hope he continues to settle in well!
:o)

Hi graham!
Welcome to fk!

You are correct, 15 litres is on the small side, but you can still have an interesting little setup!

Hydrometers are not very accurate, you will be better off getting yourself a refractometer.. But equally there's no guarantee that the water from the shop will always be at perfect salinity, always check levels in new water, and good idea to have a small pack of salt just incase, and make sure you have a container of pure RO water as you will need to top up from any evaporation with it too.

The protein skimmer won't be doing much until there is something to skim out, as you have rock in there now you will need to feed the bacteria on the rock to build up the colonies. You will see 'gunk' in the collection cup on the skimmer, which should be cleaned out regularly.

Do u have a test kit for pH, ammonia, nitrite and nitrate?

Hope that helps a little to start with, apologies if I missed anything, but I am at work answering this!

a small group of the kuhli loach would be fine, they are small wormlike fish, very placid, would not cause a problem or attract aggression from a betta.
kuhli loach are not loaches like clown loach etc, kuhli loach (Pangio kuhlii) belong to the subfamily cobitinae and clown loaches (Chromobotia macracanthus) and its closer relatives belong to the subfamily botiinae.
yep you can change filtration, the best thing to use is an internal canister filter. eheim is a reliable brand that i would recommend. i'd suggest going for one of the squarer models, as some of the newer styles like the aquaball may just stick out and take up too much room!
re test kits, you want something like this nutrafin mini master test kit (the first example containing the right tests that i found) it contains everything you need, there are a couple of other useful tests to have but you wont need them yet.

i think it would be fine. have you thought where it would go?

you could also plant either in shallow pots, and hide with rocks and gravel or just put small pebbles/rocks around the roots to prevent so much rooting around while the plants establish themselves.

problems with the swim bldder can be complicated, and to be honest swimbladder medications can only help when the problem is caused by the specific pathogens the medication targets. physical causes of swimbladder issues cant be helped by adding treatments.
unfortunately swimbladder problems are more common in fancies because of their body shape, it is more likely to be a physical problem.
can you get a pic of the lumps at all? it is possible that they could be abrasions, especially if he has beem struggling to swim upright.
a picture may give moe clues if you can get one that would be useful

hiya!
set up sounds ok so far, you said you have a 'red' bulb. this is no good for marine tanks, you need to have marine white and marine blue(actinic) if you have t8 bulbs. athough this will work, i would consider upgrading to t5 lighting when possible. and also the canister filter, this is ok as longas you clean the media at least once a week, as you dont want the gunk it collects to stay there and break down as this will add to the bioload of the tank. since you have set itup and been testing levels have you added any source of ammonia to the tank?
for now i would leave the algae, if it starts to cover evrything, give the worst rocks a scrub in a bucket outside the tank. although its not pretty, initialy its a useful visual guide to how well the tank is maturing. if one type of algae is taking over, its a sign that something is out of balance and giving it an advantage, if you have a few different types here and there that shows that algae, nutrients, and maintenance and amount of algae eaters is balanced.
any luck with the pics so far?