waterdrop
Enthusiastic "Re-Beginner"
Hi mav82uk and Welcome to TFF! 
Glad to see the other beginners and TFF members are getting you sorted out with some cycling info!
Its a shock to come here to a hobbyist site like this and suddenly start hearing so much information that seems strange, with much of it contrary to what the LFS has been doing. Its quite normal for us though, happens by the dozens here each month. Many of the hobbyists here know all these startup issues in great detail and are very keen on doing things the right way for the fish and for the future of the tank, the level of advice and detail can be quite extraordinary. In contrast, a typical LFS is in a difficult situation in that explaining to every new customer that there will be a long, somewhat complicated sounding one or two month process to go through before the aquarium filter is ready to handle fish, and that "oh, by the way, you need a somewhat expensive test kit too" would turn away customers in droves. Plus, to many older store owners, fishless cycling is a "new-fangled thing" since it started in the 1980's, and some of them feel their method from the 1920's was still good enough, if you get the picture.
Take a look at the articles in our Beginners Resource Center. In particular it really helps to learn about the Nitrogen Cycle and then how it applies to the "Fishless Cycle" and to the "Fish-In Cycling Situation."
That's what you're in with your 90L, a "Fish-In Cycling Situation" and your first urgency will be to be "the manual filter" for your fish, because you haven't yet personally tested your water, which will give you more confident information about where things sit with its progress. To be on the safe side, we'd usually recommend that you change out 50% of your water daily, until you can get your kit and test. When you change water it must be "conditioned" (treated with a dechlorination/dechloramination product per instructions) and you need to roughly temperature match the new tap water (your hand is good enough.) Meanwhile you need to find your test kit. Many of us, myself included, like and use the API Freshwater Master Test Kit, as has been mentioned by others. When you get it, post up your results for both tap water and tank water.
Once you are testing, your goal will be to find a pattern of percentage and frequency of water changes that will keep both ammonia and nitrite(NO2) from rising above 0.25ppm (parts per million) before you can get home, re-test, and be able to perform another water change. In your case, with 3 small fish in a big 90L tank, this isn't going to be too difficult a pattern. Plus, you are no doubt already part way along in the process. It usually takes about a month total and then things get a lot easier.
A new aquarium filter is really just a "kit" of hardware that must be prepared by a hobbyist who has knowledge and understands what's needed. Filters perform both mechanical and biological, and optionally, chemical, filtration. The biological part is the most important and is the unusual aspect for most beginners to the hobby. We have to grow two specific species of autotrophic bacteria, which need to build "biofilms" on the many microscopic surfaces of the "biomedia" in the filter. These two species are quite slow to grow and take some coaxing with the right conditions. Until we successfully grow them to the right colony size, the filter is not really functional.
When fish "ventilate," their gills give off ammonia. When fish waste, plant debris and excess fish food decompose in the fish tank, there are heterotrophic bacteria (different from our autotrophic bacteria) that convert these organic materials into ammonia. In nature, millions of gallons of fresh water ideally carry away any ammonia from the fish. In our tanks this isn't the case. Ammonia, even in small amounts, causes permanent gill damage and leads to shortened lives or death. In a good biofilter, one of the bacterial species (we like to call them the "A-Bacs" (for ammonia oxidizing bacteria)) will convert ammonia into nitrite(NO2) which unfortunately is also toxic. Nitrite(NO2) destroys the hemoglobin molecules in fish blood, leading to suffocation and most immediately, nerve and brain damage and then eventual shortened lives or death. Nitrite(NO2) is processed by our second species of bacteria (we call these the "N-Bacs") and Nitrate(NO3) is produced. Nitrate(NO3) is not nearly as immediately toxic and is removed by weekly water changes.
Hope this helps get you started. You can read all about it in the Resource Center, all of it is quite fascinating and part of the fun and fulfillment of the hobby.
~~waterdrop~~
Glad to see the other beginners and TFF members are getting you sorted out with some cycling info!
Its a shock to come here to a hobbyist site like this and suddenly start hearing so much information that seems strange, with much of it contrary to what the LFS has been doing. Its quite normal for us though, happens by the dozens here each month. Many of the hobbyists here know all these startup issues in great detail and are very keen on doing things the right way for the fish and for the future of the tank, the level of advice and detail can be quite extraordinary. In contrast, a typical LFS is in a difficult situation in that explaining to every new customer that there will be a long, somewhat complicated sounding one or two month process to go through before the aquarium filter is ready to handle fish, and that "oh, by the way, you need a somewhat expensive test kit too" would turn away customers in droves. Plus, to many older store owners, fishless cycling is a "new-fangled thing" since it started in the 1980's, and some of them feel their method from the 1920's was still good enough, if you get the picture.
Take a look at the articles in our Beginners Resource Center. In particular it really helps to learn about the Nitrogen Cycle and then how it applies to the "Fishless Cycle" and to the "Fish-In Cycling Situation."
That's what you're in with your 90L, a "Fish-In Cycling Situation" and your first urgency will be to be "the manual filter" for your fish, because you haven't yet personally tested your water, which will give you more confident information about where things sit with its progress. To be on the safe side, we'd usually recommend that you change out 50% of your water daily, until you can get your kit and test. When you change water it must be "conditioned" (treated with a dechlorination/dechloramination product per instructions) and you need to roughly temperature match the new tap water (your hand is good enough.) Meanwhile you need to find your test kit. Many of us, myself included, like and use the API Freshwater Master Test Kit, as has been mentioned by others. When you get it, post up your results for both tap water and tank water.
Once you are testing, your goal will be to find a pattern of percentage and frequency of water changes that will keep both ammonia and nitrite(NO2) from rising above 0.25ppm (parts per million) before you can get home, re-test, and be able to perform another water change. In your case, with 3 small fish in a big 90L tank, this isn't going to be too difficult a pattern. Plus, you are no doubt already part way along in the process. It usually takes about a month total and then things get a lot easier.
A new aquarium filter is really just a "kit" of hardware that must be prepared by a hobbyist who has knowledge and understands what's needed. Filters perform both mechanical and biological, and optionally, chemical, filtration. The biological part is the most important and is the unusual aspect for most beginners to the hobby. We have to grow two specific species of autotrophic bacteria, which need to build "biofilms" on the many microscopic surfaces of the "biomedia" in the filter. These two species are quite slow to grow and take some coaxing with the right conditions. Until we successfully grow them to the right colony size, the filter is not really functional.
When fish "ventilate," their gills give off ammonia. When fish waste, plant debris and excess fish food decompose in the fish tank, there are heterotrophic bacteria (different from our autotrophic bacteria) that convert these organic materials into ammonia. In nature, millions of gallons of fresh water ideally carry away any ammonia from the fish. In our tanks this isn't the case. Ammonia, even in small amounts, causes permanent gill damage and leads to shortened lives or death. In a good biofilter, one of the bacterial species (we like to call them the "A-Bacs" (for ammonia oxidizing bacteria)) will convert ammonia into nitrite(NO2) which unfortunately is also toxic. Nitrite(NO2) destroys the hemoglobin molecules in fish blood, leading to suffocation and most immediately, nerve and brain damage and then eventual shortened lives or death. Nitrite(NO2) is processed by our second species of bacteria (we call these the "N-Bacs") and Nitrate(NO3) is produced. Nitrate(NO3) is not nearly as immediately toxic and is removed by weekly water changes.
Hope this helps get you started. You can read all about it in the Resource Center, all of it is quite fascinating and part of the fun and fulfillment of the hobby.
~~waterdrop~~
