This answer might be longer than you wished, but I thought I would offer some advice to allow you to stock your tank even though you have had some troubles with your cycle.
Most community fish should be fine together, of course some better than others. My advice would be to have some fun making your selections. I just wanted to offer some advice on the stocking itself. I would but some bacterial stocks, Stress Zyme or another bacterial additive. These bacteria are essential for balancing the nitrogen cycle no matter how large or small your bioload is. Because by adding more fish to your tank you will be also adding more waste, you should balance this accordingly. I used this advice when I started from a very experienced friend and it has never failed.
Since you are considering adding 15 more fish or more eventually to your tank this is very important. You will invariably get an ammonia spike from all the new waste, which means a new nitrogen cycle start, possible a big one. It sounds like you have had trouble with this and it is of course something to be avoided. Your 55gal can certainly handle the bioload of 20+ small fish, but that is dependant on the established bacterial colony present.
Too much ammonia does not necessarily mean a correlating amount of bacteria will consume it. Also excess ammonia beyond the 'minimal' needs for feeding and maintaining the bacteria produces extra 'waste' nitrites plus left over ammonia they could not process... much like feeding more food to fish than they can consume will mean decaying food on bottom and ammonia spikes from this excess, too much ammonia being feed to bacteria means they can out put more nitrites with excess ammonia unused left behind.
In the beginning of re-cycling your tank again, a proper 2 reactive tester will show ALL combined ammonia levels (API 2 part ammonia tester registers non toxic levels and toxic as one reading, a one part drop tester of another brand like Nutrifin will only show toxic ammonia (check boxes for type of ammonia a brand tests for)...use this to your advantage to read what’s up in your tank… minus toxic readings from API test, that’s the non toxic amount present however use overall ammonia present on API tests as guide to cycling and be conscious of the toxic levels always, of course and address amount of ammo lock next used at toxic ppm levels for dosing only, usually adjust and dose every second day of cycling for toxic ammonia.) Never use testing strips to gauge water quality in a cycling period as they are too unreliable due to the range of ppm sensitivity being vast. Use liquid drop testers.
Do not exceed 1.5 to 2.0 ppm maximum of the combined ammonia readings : where ammonia present is primarily non toxic and less than 0.6 ppm being the sum of daily occuring toxic form between testing. Ammo lock the toxic portion every second day and at WC day to arrive at a total non toxic amount ,after confirming test, of between 1.5 -2.0 to get those bacteria growing. Non toxic Ammonia will feed bacteria and encourage the cycling to begin while protecting fish health. These levels will be high 'only' during the ammonia phase of the cycle. After nitrties begin, stop the ammo lock in favour of frequent WC's, move to regular habit of cleaning up leftover fish food not eaten in 5 to 10 minutes and clean that gravel every 3 days still. This will assist bacteria to create a stable ammonia testing level of 0 ppm fairly. Nitrites will reduce when nitrates arrive but be viligant to keep these numbers down during this phase of cycling. Remember ammonia cycling continues for as long as it has to until nitrates show while ammonia decreases. These suggestions I have given only increase the process of bacteria establishment in newly cycling/recycling tanks.
You will need to water change every 3 to 4 days at 25% in a stocked fish tank during cycling, leaving all fish food after feeding and waste IN tank until gravel cleaning at day 3. This is to help keep desired ammonia levels stable since in this particular case, you want ammonia present... just a certain amount though. So test and dose 1/2 hour after water change. ( I take an extra step and measure before and after WC to see how much ammonia my tank regime is encouraging, how the WC affects this and adjust WC frequency accordingly) Use a good water conditioner that does not also 'lock up' ammonia ( use one method to address ammonia, easier to monitor and change levels).
However...***Less 'overall' ppm is required if only few bacteria present ( as usually the case in beginning, use the 2 different tests above to guide you as to overall levels required at any given time. (In the beginning, just ammo lock levels up to 1.5ppm on average before first water change in this regime checking every 2 days)
After this, you want 'low' increases in toxic ammonia at your 2 day testings and before water changes. Dose toxic ammonia portion of ppm, every second day and 'after' a water change, if testing shows it is still required... all the while making sure not to exceed overall ammonia ppm target. The amount of toxic ammonia indicates bacteria colony size/ability to control it under your tanks conditions. More toxins produced, the less bacteria established to handle it if you have been cleaning and feeding tank appropriately. They are consuming as much non toxic as they can in between and connot eat new toxic ammonia additionally being created daily. Use this to imagine, if you will, the living size and capacity of the nitrificaying bacteria ( name them... makes them seem more like the living animals that they are.. just like your fish and remind you to care and feed them in the same way... and remember they will increase over time and consume more eventually maxing out thier colony size at a level that supports them in the tank ecosystem)
Allow overall ammonia levels to increase when needed through ‘less volume’ in water changes (20% WC) if following above instructions but do NOT do less ‘water changes’ during cycling period. Conversley, any overall non toxic ammonia levels exceeding 1-1.5 ppm means a larger water change ( up to 50 % max) then go back to usual 25% change every other day or adjust this amount to say 30% if your tank regularly creates more ammonia than 25% WC can regulate consistantly at the above max. rates.
Add bacteria faithfully and daily for the first week at full dose per tank size, then every other day about 1/4 as much if using Seachem’s or other live bacteria product such as Stress Zyme as you help them along to establish themselves until nitrite cycling begins. Make sure not to clean sponge or change bioballs holding bacteria during cycling and then alternate changing sponge and bio balls every month there after to keep bacteria always in tank system. Rinse off debries 'lightly' in the tank's water and net the floaters. Only change carbon every 2 weeks during cycling. Clear tanks are not the norm in cycling and will clear on thier own, don't use a water clearer product during this time other than carbon filter.
Not all bacteria introduced will flourish initially so its a delicate balance replenishing them and helping them along with non toxic ammonia feed but not killing your fish while re- cycling.
Once cycled, ammonia readings of 0pmm are achieved not because there is no ammonia in the tank but rather the bacteria consume it as it occurs keeping the readings low. Depending on how many bacteria you have to support vrs how much ammonia is produced in your tank environment daily, is the balance that must be maintained. Too much ammonia from lack of maintenance, decaying plants where their nutrient needs are also not being met, type and number of fish ( goldfish expel more ammonia aside from their waste) total fish waste produced and left over decaying food matter contribute to affect general amount of ammonia production in the tank which can be influenced further by higher ph levels and temp. Weekly testing of water is always required at a minimum to off set ph and temp variables on these factors. *Test every other day when cycling.
So keeping bacteria fed is same as with fish... it is after all a comlete ecosystem you are keeping here, so only feed enough to do the job and nothing more so the tank functions holisticly as a proper enviroment, often maintaining itself well with little intervention.
This natural process is greatly helped by light gravel cleaning once every 3 days to remove decay, take off yellowing leaves off plants at this time ( mimicing natural water flow in the wild for your fish that flushes toxins, debris and wastes away... this is different in water current strength, temp, lighting and ph for every fish so keep simialr needing fish AND plants together in a tank only or loose your inhabitants.)
Provide extra sources of air for fish if you have no live plants (no such thing as too much air for any enviroment requiring it). Cycle lighting to approach normal environments for fish and 25% water changes every 2 days, then weekly and then bi monthly to monthly as tank matures and becomes established. Adding bacteria at water changes ia always a good idea… wait 15 minutes after adding conditioned water to filtered before doing so.
I hope this helps you establish the perfect balance for your new residents. With new guys and new babies abounding I am sure you will see some fluctuations in your test readouts. I went though many a frustrating start to a tank. I have chronicled my knowledge here to save you some frustrations hopefully.
