Mystery 'bubble nest'

[kyektulu]

I am wondering if anyone can help me explain the sudden arrival of something which I can only describe as a 'bubble nest' in my tank.

I have a 3ft bow fronted tank which has been up and running for around a year, the water PH, Nitrate, Nitrite and Kh are all perfect and I use living plants.
In the tank I have 2 Senegal Bichirs, several Cherry Barbs, White Cloud Mountain Minnows and three Dwarf Hoplo's.
All of which I am quite certain do not breed by bubble nests, I have bred both the Barbs and Minnows and have bred other species of Catfish in the past so these could not of been the potential parents.
I have checked up online and although not much is known about the breeding of Senegal Bichirs I am quite sure that they do not breed by bubble nest.

I am quite confused as I have been keeping tropical fish for many years now and the only other occurence of a bubble nest is from my Dwarf Gouramis breeding...

Is there anything at all that could explain the sudden appearance of this 'nest'?

 

[Puriti]

Could be your filter, mine does that sometimes, or if you added anything to the tank recently sometimes will do it (Bettafix does that to me sometimes) ...other than that ._. no idea

 

[kyektulu]

Hmm I doubt it is the filter, I have had the same filter in for a long time and the filter media is changed frequently.
I have not added anything to the tank recently...

Thanks for the reply.

 

[Toirtis]

All Hoplos are bubble nest breeders, so it is very likely theirs....and no, Polypterids do not spawn with bubble nests.