Ok stop. The reason your fish died was not the 'additive' - that is likely de-chlorinator.
Before I go on to explain about de-chlorinator and cycling and so on... What size tank do you have (I mean do you know the size in gallons - or can someone else on this board convert the measurements for me?
)? Also, can you describe the 'sucking loaches'? These are probably what is commonly called a CHINESE 'algae-eater'. They get to 10" and will become very aggressive and fish-eaters as they mature. You should return the remaining loach to your local fish store and don't replace it with any fish just yet. Also, what kind of tetras do you have? If you don't know the species, a quick discription should be enough to identify them.
De-chlorinator MUST be added in the right amount (read instructions on bottle) at each water change and it must be added to the tap water BEFORE you put it in the tank.
The reason de-chlorinator is so important is that tap water contains chlorine. Chlorine is harmful to fish. Chlorine will also kill good bacteria. This brings me on to cycling.
Cycling is a period all new tanks must go through. Fish produce waste. This waste contains a deadly toxin called ammonia. Thankfuly, there are good bacteria that live in your filter and gravel that eat this ammonia and then produce nitrIte. Unfortunately, nitrIte is also dangerous to fish. Naturaly though, there are bacteria that can use this nitrIte up. These good bacteria then change the nitrIte to nitrAte. NitrAte isn't as harmful. In fact, it is relatively safe if you keep it under control.
To keep nitrAte under control, you need to do a weekly partial water change with DE-CHLORINATED water of about 25%. As long as you don't add too many fish and don't feed too much and keep up with maintainance, a cycled tank will always be healthy.
A 'cycled' tank is one with all the good bacteria growing in balance. In a cycled tank there is no ammonia and no nitrIte and just a little bit of nitrAte. You need to buy a test kit from your local pet store and test these levels.
Because your tank is new and you have added lots of fish too quickly, the good bacteria have not had a chance to grow yet. Slowly the ammonia-eating bacteria will start to multiply, then ammonia will go down but nitrIte will start going up. Once the nitrIte-eating bacteria start growing, the nitrIte will also go down and nitrAte will go up. Every time the level goes up, it is called a 'spike'. The ammonia and nitrIte spikes are very dangerous for your fish.
An ammonia spike is what killed your fish.
For the next 6 weeks or so, your tank will still be in the process of cycling. If you want to keep the fish alive, you need to get a test kit for ammonia and nitrIte and nitrAte and take back the 'sucking loach'. Also buy a gravel vacuum and a bucket or two for water changes so you can de-chlorinate the water before you put it in the tank.
When you do a water change you need to first vacuum your gravel a little to get rid of extra debris and then replace about 25% of your tank's water with de-chlorinated tap water of a similar temperature.
Sometimes you may need to clean out your filter too. Always do this during a water change. Take the filter media out (the sponge or floss inside your filter) and rinse it or squeeze it gently in old tank water that you have just taken out of your tank. NEVER clean it under tap water or any water that contains chlorine. Chlorine kills the good bacteria that live in your filter media and are realy important like I said before. If you ever need to change your filter media (if it starts to degrade) take only part out at a time and replace it over a period of a few weeks. That way you don't take out ALL the bacteria at once. That would kill your fish because you would get an ammonia spike.
Because you are still in the cycling process. You probably have lots of ammonia in the tank right now. Because of this, you need to do a water change every 2 days for about 3 weeks. I know it's a lot but your fish WILL die otherwise. Make sure you work out how much de-chlorinator you need to add every time you change water so that you don't kill the fish or bacteria with the chlorine in your tap water. This is why you need to know how many gallons your tank is and get a bucket so you can measure how much you put in or take out (know how much the bucket holds).
You will need to continue doing water changes every few days until your tank is completely cycled. You will know when this is because you are going to get a test kit and test your water every other day. When ammonia and nitrIte are 0, you can reduce your water changes to just one weekly. Do a gravel vacuum at the same time.
Keep in mind that rotting food and debris also increases the ammonia levels, and therefor the nitrIte and nitrAte levels, so keep feeding to a minnimum while the tank is cycling and don't over-feed later on either.
To speed up the cycle, get some gravel or filter media from an established tank and put it in yours. This will already contain some good bacteria and will make your tank get the good bacteria growing much faster.
Hmm.. What else? Ah yes
- read through the first three links in my signature. The fishless cycling link won't be of much use now but it may give you an insight into what is usualy the best way to cycle (BEFORE you get fish).
Good luck!
Oh and I forgot to answer your last question
- You obviously should not add any more fish for quite a while yet. Once your tank is fully cycled, you can add a few SMALL-GROWING fish a couple at a time every week or so. HOWEVER, without your tank size in gallons (or litres if you preffer) I can't say just how many more fish will live happily in your tank
Always make sure you know what fish you are buying and research them first. There is a very good and constantly expanding fish index on this board but fishbase.org and google.com will work for those species that have yet to be covered here. NEVER buy a fish without knowing how it'll be with the fish you already have, what it eats, its water requirements and how big it will get.
There is a guideline for tropical fish that says 'one inch of fish per gallon of water'. 'Inch of fish' reffers to the adult size not including the tail. So your tiger barbs, for example, each count as 2" of fish. 'Per gallon of water' reffers to every US gallon (as opposed to imperial gallons) that is actualy water. What I mean by this is that a 10 gallon tank, for example, only holds about 8 gallons if it has lots of ornaments, a deep gravel substrate and isn't filled completely to the rim... What all this amounts to is that 4 tiger barbs need at least 8 gallons of water to thrive.
The problem with this guideline is that it only applies to small fish (typicaly under 4") and that behaviour and requirements are not taken into acount. Some fish can be very aggressive - for example you can't keep two 2" male bettas in 4 gallons of water - they will kill each other - and you can't keep zebra danios in anything less than 20 gallons (though they only get to 1.5") because they need space to swim and are very active. Then there's fish, like your barbs, that should not be kept with fish that have long fins because they nip and they should be in groups of 7 or more to minnimize aggression (don't worry about this until after the cycle). Something similar applies to schooling fish like most tetras, your tiger barbs, the zebra danios and others which need to be in groups of at least 6,7 and 5 respectively (as a minnimum).
You get the idea - research is key.
