Ask for advice...and then proclaim that you intend to do exactly what everyone tells you not to do. Less than 24 hours ago you said:
...but here we go again. Well, there's good news. You will prove yourself correct. Put both fish in; eventually, one or both will be dead. You will then have proven that, by definition, both can be kept together "for life". Nice work; what a shame that nobody warned you ahead of time.
Ya know what they call folks who constantly ask for advice, and then do the exact opposite thing? Askholes.